OPTICAL TRANSMITTER COMPRISING A STEPWISE TUNABLE LASER

Abstract: A laser, useful as part of a multiple wavelength optical fiber transmitter, includes a single mode, multiple quantum well semiconductor amplifying mirror as a gain element an includes a second mirror to form an optical cavity, the length of which defines a sequence of optical modes. The gain element has an arrangement for tuning the laser from one mode to another by altering a temperature of the amplifying mirror.

Description: OPTICAL TRANSMITTER COMPRISING A STEPWISE TUNABLE LASER Field of the Invention The present invention relates to lasers emitting single longitudinal and traverse mode radiation at selected wavelengths defined by a frequency comb, in particular stepwise tunable external cavity surface emitting semiconductor lasers pumped optically or electronically for use in spectroscopy, process control and optical communications. The lasers are advantageously frequency stable and rapidly tunable. Background of the Invention The use of a multiplicity of single frequency lasers for optical channel switching in a fiber optic transport network is known. Current switching systems using single frequency lasers require extremely complex circuitry. Laser based optical channel switching would be enhanced if the laser: (1) is rapidly tunable; (2) provides random access to any particular desired frequency; (3) is reliable and consistent in its tuned output frequency ; (4) is consistently receptive to an input signal; and (5) is substantially uniform output power independent of the output frequency. Methods of selecting a single mode and adjusting laser frequency are described in text books such as A. Yariv, Quantum Electronics. John Wiley and Sons, New York, 2 nd edition, 1975 and M. D. Levenson and S.S. Kano. Introduction to Nonlinear Laser Spectroscopy. Revised Edition, Academic Press, San Diego, 1988 and Demtroder, Laser Spectroscopy. Springer, Berlin, 1996. However, rapid frequency switching without lasing at intermediate frequencies is difficult without, for example, altering some other laser parameter. Dense wavelength division multiplexing (DWDM) for optical fiber telecommunications applications requires optical transmitters having lasers that can be tuned to any frequency in the standard International Telecommunications Union (ITU) grid (wavelength comb) with a relative frequency error not greater than ten percent of the ITU channel spacing. Such control of the lasing frequency cannot be achieved with existing distributed feedback diode (DFB) lasers without complex electronic control involving compensation algorithms to handle DFB ageing. It is desirable that a single laser, or at most a few lasers, cover all of the DWDM channels, and that it can be reliably and reproducibly set to any channel frequency. Current lasers have only a limited tuning range, which covers only a fraction of the full ITU grid. Known DFB lasers have limited tunability; and the temperature tuning coefficient of telecom DFB lasers is typically 0.09nm/°C. For a DFB laser thermal operating range of ± 20°C, or 40°C total temperature differential, one DFB laser could be expected to cover only a wavelength range of 3.6nm (or about 460 GHz, representing only four channel coverage with 100 GHz channel spacing or 36 channel coverage with 25GHz spacing) even if the necessary accuracy in wavelength selection could be achieved. In addition, DFB lasers have only about 35 to 45 dB of side mode suppression. If side modes are not sufficiently controlled, the laser may excite adjacent communications channels, resulting in interference. Because of these drawbacks, the telecommunications industry has recently turned to vertical cavity surface-emitting lasers (NCSELs). Micro-cavity VCSELs include semiconductor structures which have multiple layers epitaxially grown thereon, typically Gallium Arsenide or Indium Phosphide. The layers comprise semiconductor or dielectric Bragg mirrors which sandwich layers comprising quantum well active regions. Photons emitted by the quantum wells are reflected between the mirrors and then are emitted vertically from the wafer surface. NCSELs naturally have a circular dot (beam) geometry with lateral dimensions of a few microns. The narrow emitting aperture facilitates direct-coupling to optical fibers, because it typically supports only a single lateral mode (TEMoo), but is sufficiently wide to provide an emerging optical beam with a relatively small diffraction angle. The semi-conductor structures used in external cavity lasers have been typically Fabry-Perot devices having one facet antireflection coated so as not to interfere with external cavity operation. However, the gain medium can also use a semiconductor optical amplifier (SOA), which is typically an edge-emitting semiconductor gain structure. A semiconductor optical amplifier (SOA) is a device that amplifies an input signal normally of optical origin. The amplification factor is typically high (>100 or 20 dB). An SOA amplifies light, as it propagates through a waveguide made of semiconductor material, through stimulated emission (just as a laser produces radiation). In essence, an SOA is a Fabry-Perot device without feedback, having optical gain when the amplifier is pumped (optically or electrically) to create a population inversion leading to stimulated emission. Feedback requires two mirrors oriented substantially perpendicular to the propagation direction of the light and SOAs are possible in which one surface of the chip is perpendicular and the other is not, thus avoiding feedback even though one surface can act as a mirror. In both VCSEL and SOA cases, one creates a so-called "active mirror" that defines one of the laser cavity reflectors but also provides the gain medium for the laser. The gain in this active mirror results from either electrical or optical excitation (pumping) of carriers that recombine in quantum wells to create photons. The external cavity defines the coherence properties (wavelength) of these photons. If the gain medium is half a NCSEL, the external cavity version is referred to as a vertical external (or extended) cavity surface-emitting laser (NECSEL). If the gain medium is an SOA the laser is called ECSAL (External Cavity Semiconductor Amplifier Laser). Each of these types of gain medium may be employed in any of the suitable embodiments of the invention disclosed herein. In general, we will refer to both such types of laser as a STECAM (Stepwise Tunable, External Cavity, Active Mirror) laser. A NECSEL based active mirror is usually an epitaxially grown semiconductor, typically a few microns thick, which comprises a multiple quantum well active substrate gain region sandwiched between a Bragg mirror grown on the semiconductor and a capping layer. The active mirror may also have an antireflection coating that is either epitaxially grown or dielectrically deposited. An external cavity is then formed by a separate, second, passive mirror that forms a stable resonator with the active mirror. Such -an external cavity can either be a high reflectivity dielectric boricave mirror or a piano/piano mirror with an intracavity refocusing element such as a lens. ' See WO 00/10234, "Optically-Pumped External-Mirror Vertical-Cavity Semicόndύc ' tor-Laser", which disclosure is incorporated herein by reference. A drawback 'of that NECSEL is the absence of any wavelength tuning mechanism. Other VECSELs are described in a paper by Sandusky and Brueck, entitled: "A CW External-Cavity Surface-Emitting Laser", IEEE Photonics Tech. Ltrs., Vol. 8, No. 3, March 1996, pp. 313-315; and, in a paper by Kuznetsov, Hamimi, Sprague, and Mooradian, entitled: "High Power (>0.5-W CW) Diode-Pumped Vertical-External- Cavity Surface-Emitting Semiconductor Lasers with Circular TEMoo Beams", IEEE Photonics Tech. Ltrs., Vol. 9, No. 8, August 1997, pp. 1063-1065. Co-inventors Garnache and Kachanov of the present invention have previously reported in a note entitled "High-sensitivity intracavity laser absorption spectroscopy with vertical-external-cavity surface-emitting semiconductor lasers", Optics Letters. Vol. 24, No. 12, June 15, 1999, pp. 826-828, that an optically pumped multiple- quantum-well ("MQW") VECSEL is an excellent candidate for use in high sensitivity intracavity laser absorption spectroscopy (ICLAS). In the ICLAS method an absorbent analyte is placed inside an external cavity of a broadband laser with homogeneously broadened gain. See also Garnache, Kachanov, Stoeckel and Houdre: "Diode-Pumped Broadband Vertical-External-Cavity Surface-Emitting Semiconductor Laser: Application to High Sensitivity Intracavity Laser Absorption Spectroscopy", JOSA-B-B, Vol. 17, No. 9, September 2000, pp. 1589-1598. The disclosures of these two articles are incorporated herein by reference. An intra-cavity etalon and a Lyot filter were said by Holm et al. to stabilize VECSEL radiation at a single wavelength in "Actively Stabilized Single-Frequency Vertical-External-Cavity AlGaAs Laser", IEEE Photonics Technology Letters, Vol. 11, No. 12, December 1999. U.S. Patent No. 5,668,900, the disclosure of which is incorporated herein by reference, discloses a vertical coupler filter. One approach for tuning a VECSEL is described in a note by D. Vakhshoori, P. Tayebati, Chih-Cheng Lu, M. Azimi, P. Wang, Jiang-Huai Zhou and E. Canoglu entitled, "2mW CW single mode operation of a tunable 1550nm vertical cavity surface emitting laser. with 50 nm tuning range", published in Electronics Letters, Vol. 35, No. 11, May 27, 1999, pp. 900-901, the disclosure thereof being incorporated herein by reference. Cavity length can be changed by applying a potential difference between a dielectric membrane and an ambient supporting structure, thereby applying ah electrostatic force to the membrane mirror and causing its curvature (and hence the cavity length) to dha'nge. Changing the cavity length shifts the cavity resonance frequency which results in laser frequency tuning. However, this VECSEL does not appear to meet DWDM telecom requirements. 1 The micro-machined membrane mirror must be flexible in order to move the required tuning- distance, and is therefore sensitive to external perturbations arid can ' becohie self-excited. It is also complex to produce. Furthermore, a complex feedback control system would be required to maintain membrane mirror position, limiting absolute frequency set point stability and reproducibility in laser tuning. Caprara et. al. (e.g. U.S. Patents Nos. 5,991,318; 6,097,742 and 6,167,068) have described a very large, high-power VECSEL requiring mechanical rotation of a tuning element. Tuning is therefore slow and causes energy to build up in the successive modes traversed by the filter. Telle and Tang , in Applied Physics Letters 24, 85-87 (1974), have described an electro-optic frequency selective filter for dye lasers that might be capable of rapid tuning if a sufficiently high voltage can be sufficiently rapidly applied. However, the multi-kilovolt potentials required are too high for practical telecommunication use, and the beam collimation required is not compatible with VECSEL type lasers. Other previously known tunable filter technologies have too much loss for use with surface emitting semiconductor gain media and/or they transmit extra unacceptable frequencies. However, when used in conjunction with a higher gain medium such as an SOA and appropriate electro-optical materials such as liquid crystals, these filters (e.g. etalons or Lyot filters) can provide suitable lower voltage tuning. It is apparent therefore that a need remains for a simplified, reproducible and widely tunable single mode MQW VECSEL or SOA based laser for optical fiber telecommunications. Especially, there remains a need for a compact near infra-red laser system capable of switching quickly (<10 sec) among cavity modes spaced at -25-50 GHz from one another without producing unwanted frequencies. Summary of the Invention We have designed a laser which, in the most preferred embodiments, has the following properties. An optical fiber transmitter module includes a single mode MQW VECSEL having a semiconductor structure with a homogeneously broadened active gain region and an external mirror spaced from the semiconductor structure by a spacer such that a cavity length is in a range of 0.5 mm and 50 mm and is chosen to create a laser frequency comb corresponding to a predetermined optical channel spacing arrangement. Thus, in one embodiment the present invention provides an external cavity laser (preferably having a gain greater than cavity loss over a wavelength band of at least 5nm at a specific temperature) and comprising: (a) a first mirror, preferably having at least 90 percent reflectance; (b) a homogeneously broadened active gain region, optionally integral with the first mirror (and preferably fabricated monolithically therewith), and comprising multiple quantum wells and having a length equal to at least one design wavelength of the laser; (c) an external mirror (preferably fixed at such a position that at least some of the potential modes of oscillation of the laser correspond to pre-set frequencies, such as those of the ITU grid); and one or both of (d) an intra-cavity, optionally electronically-actuated, frequency-selective element; and (e) a spacer for locating the mirror (c) with respect to mirror (a) and gain region (b) at a distance therebetween to define the external cavity, the cavity length preferably being selected to create, for example, a laser frequency comb corresponding to predetermined optical channel spacing, such as that of the ITU or other telecommunications grid. Preferably the laser has both the frequency-selective element (d) and spacer (e). The invention also provides an external cavity laser having intra-cavity optics, the laser operating in single transverse mode, and at least a sub-set of longitudinal modes corresponds to predetermined frequencies. The predetermined frequencies may be those of a grid for use in telecommunications, such as the ITU grid. The strength of emission on sidebands is no more than 0.001 times that of the main mode Element (d) may serve to induce loss for all but one potential mode of oscillation, thereby suppressing lasing on the loss induced modes, and may switch ifrόm one unsup'pressed ihode to another, thereby providing frequency switched laser emission. Element (d) may comprise one or more electro-optically tunable Lyot filter arid/b'r one or more eiectro-optically actuated etalons and/or one or more electro- optically tunable ve'rϋieal coupler filters. With regard to Lyot filters, reference may be made to Polarization, edited by Bruce Billings 1990, ISBN 0-8194-0495-0, and to B Lyot, Comptes Rendus, 1933, the disclosures of which are incorporated herein by reference. A MQW VECSEL or other semiconductor structure involved in the present invention- can beTormed by molecular beam epitaxy or metal oxide chemical vapor deposition for example in a manner enabling removal of the semiconductor substrate, thereby overcoming limitations and drawbacks of prior approaches in which the substrate contributed to the presence of a Fabry-Perot etalon or other unwanted optical element. An optical fiber transmitter module can include a SOA with homogeneously broadened and unpolarized active gain and an external mirror. Reference may be made to Physics Today, September 2000, page 30, the disclosure of which is incorporated herein by reference. A laser can have reproducible absolute emission wavelengths that correspond to standardized wavelength division multiplex (WDM) channel wavelengths, as used in optical fiber telecommunications networks, such that the laser steps from channel to channel and its emission wavelengths match any desired channel wavelength accurately and have channel separation with an accuracy better than ten percent of channel spacing. Side mode suppression can be in excess of 40 dB. A VECSEL or ECSAL can be used as a laser source within a wide variety of applications and environments including telecommunications test equipment. A compact VECSEL or ECSAL can have axial cavity modes (i.e. axial mode frequencies) which correspond to pre-determined communications or spectroscopic channels, and can be capable of randomly switching among such channels in 1 millisecond or less. A single ECSAL can access the entire C or L optical communications band. A frequency agile laser module can meet all current requirements for DWDM optical fiber communications. An external cavity type laser can have a fixed cavity length selected so that permitted lasing modes match desired emission frequencies (e.g., the frequencies of •the ITU grid) in accordance with the formula πc/L=12.5 GHz where L is the cavity length and c is the velocity of light between the mirrors which form the cavity. A compact MQW based optical transmitter can have cavity modes which correspond to pre-determined communications or spectroscopic channels. An optical transmitter can emit a stable TEMoo beam at the frequency of a specific channel and can be switched to another such channel by changing some convenient control parameter. An optical transmitter randomly can switch among channels in 0J millisecond or less. A preferred laser includes an intra-cavity, fast electro-optic tuning element providing minimum optical loss only at selected frequencies. A preferred laser also includes a gain medium that is homogeneously broadened, coupled to a circular (Gaussian) external laser mode. An optical fiber transmitter comprises an active mirror for emitting an information-carrying laser beam at a design wavelength and has an external cavity length defining a plurality of " optical modes, each mode corresponding to a channel wavelength of a multichannel optical telecommunications system. A stepwise tunable external cavity amplifying mirror laser (STECAML) containing an active mirror such as the semiconductor structure of an optical-pump-excited VECSEL or an SOA, has a homogeneously broadened MQW active region wherein the net gain curve exceeds cavity losses over a band which is less than mode-to-mode spacing, the gain region being tunable to step from a first mode to an adjacent or any other selected second mode and to remain stable at the second mode. A tuner tunes the laser from mode to mode thereby to select any of the desired channels. A conventional external optical amplitude modulator adds user traffic to a beam from the laser to provide an information-carrying laser beam, and a coupler couples that beam into an optical fiber. In the case of an SOA, the structure of the gain medium may allow integration of a modulator or interferometer (e.g., electro-absorptive or Mach-Zehnder). The external cavity length is determined according to: the frequency spacing of the ITU grid (or whole number fraction thereof) to be achieved; the optical and/or temporal dispersion produced by intracavity optical elements; and the effective optical and/or temporal dispersion caused by tuning. In either a 1 VECSEL or SOA based laser the external mirror is positioned relative to the semiconductor structure by a spacer mounted to (preferably directly or indirectly on) the heat sink at a distance, for example, of 0.5 to 50 mm to form the external cavity and chosen to create a laser frequency comb corresponding to a predetermined optical channel spacing. Alternatively, a focusing intra-cavity lens and plano/plano mirror can be used to form the external cavity. The external cavity design should be a stable resonator as defined in A.E. Siegmann, Lasers. University Science Books, 1986, where the external. cavity provides feedback stabilization. In some embodiments, the VECSEL or ECSAL includes a heat sink and a semiconductor structure grown by molecular beam epitaxy, vapour deposition or other suitable technique upon a substrate. The semiconductor structure may be attached to the heat sink. The semiconductor structure may comprises a multi-layer semiconductor or dielectic mirror region, a homogeneously broadened MQW gain region having a thickness equal to at least one design wavelength, each quantum well being optimally positioned with respect to a standing wave in the active gain region at the design wavelength, and an antireflection coating or other region having a low reflectance at the desigri wavelength. In the case of a MQW VECSEL the mirror may be a semiconductor Bragg reflector achieving at least 99 percent reflectance and the active region can be as short as one design wavelength. Where the gain region comprises a SOA, one mirror of the 1 cavity can be a cleaved facet that may have a dielectric coating and its active region may be hundreds of times the design wavelength. The active region is normally electrically pumped. However, the waveguide design must be such that good overlap between the external cavity mode and the fundamental waveguide mode is achieved and that higher order waveguide modes are not excited. In the case of an optically pumped MQW VECSEL, an in-plane laser (e.g. Fabry Perot or DFB diode laser) providing pump radiation is aligned relative to an external surface of the semiconductor quantum well at Brewster's angle relative to the axis of pump laser efriission. The diode laser pump may be a sub-assembly which is aligned and secured in a sidewall of the spacer structure and may thereby made integral therewith. A pump radiation absorption element or aperture is preferably formed inJhe spacer-structure at Brewster's angle opposite an angle of incidence of the punip radiation for absorbing any pump radiation residually reflected from the external ' surface of the-active region. By pumping the structure at Brewster's angle, over 90 percent of the pump radiation is typically absorbed. An optimized pumping arrangement can deliver as much as 99 percent of the pump radiation to the VECSEL. Brewster's angle is about 74 for InP based materials, which corresponds to a tan(74) = 3:1 ratio for the incoming pump beam axes to produce a circular spot on the ' semiconductor. Because the geometry of most edge-emitting devices is 3:1 , pumping at Brewster's angle eliminates the need to circularize the pump laser beam, reducing the complexity of the pump optics. In fact, only focusing optics such as one or two lenses are required to image the pump beam to the correct spot size on the active region. A simple λ/2 plate provides the correct polarization for incidence at Brewster's angle and hence maximum absorption. As is known, the term Brewster's angle refers to the angle of incidence of light reflected from the surface of dielectric material at which the reflectivity for light whose electrical vector is in the plane of incidence becomes zero. This is sometimes referred to as the polarizing angle. In the present invention where the dielectric material has a multilayer structure and a composite refractive index, Brewster's angle (sometimes referred to as an angle equivalent to Brewster's angle) is the angle corresponding to that composite refractive index. The size of the pump beam on the VECSEL is advantageously matched to the size of the external cavity transverse mode to account for thermal effects (such as thermal lensing) in the VECSEL itself. A method for calibrating a STECAML is also provided. The STECAML includes an external cavity of a length determined by an external mirror fixed by a spacer. The length defines a plurality of optical modes, each mode corresponding to a channel wavelength of a telecommunications system. The semiconductor structure of the active mirror (region) has a homogeneously broadened MQW active region wherein the net gain curve exceeds cavity losses over a band which is less than the mode-to-mode spacing, the laser output being tunable to step from a first mode to an adjacent second mode'and to remain stably at the second mode. A tuning mechanism such as' a thermoelectric cooler for cooling and heating the active gain region, or a frequency-selective element such as an intra-cavity etalon, is provided for tuning the STECAM laser from mode to mode. A digital controller including a wavelength- selective optical sensor responsive to laser output tunes the laser from mode to mode and maintains the laser at each mode. The wavelength-selective optical sensor generates pulses responsive to inter-mode optical transitions. The method includes steps of: sweeping 'the tuning mechanism between the longest and shortest wavelength modes capable of being generated by the laser in. accordance with a control parameter generated by the digital controller, recording in the memory of the digital controller a transition control parameter presently being put out by the wavelength-selective optical sensing means upon detection of pulses responsive to inter-mode optical transitions, and < it o determining and recording single mode set values as approximately half increment magnitudes between magnitudes of adjacent recorded transition control parameters: Thus, the precision required of the tuning mechanism is less than that necessary for exact landing on an ITU channel wavelength, and instead merely that required to create a profile such that the laser snaps itself to the appropriate channel, whose wavelength has already been predetermined by the external cavity. Such a laser does not require expensive wavelength control circuitry. Brief Description of the Drawings The invention is further illustrated with reference to the accompanying drawings. Each drawing independently illustrates a novel and inventive structure, which independently of other features referred to herein defines an embodiment of the invention. Figure 1 is a pair of graphs of gain intensity as a function of VECSEL optical mode, showing an initial higher gain and a residual steady state lower gain. Figure 2 shows thermally induced mode hopping over extended time intervals. Figure 3 is a schematic diagram of-anoptical fiber transmitter unit including an optically pumped MQW VECSEL. Figure 4 is a' band gap energy diagram superimposed upon a diagrammatic cross section of an epitaxially-grown semiconductor structure. •Figure 5 is- a graph of reflectivity (upper trace) and photo-luminescence (PL) (lower trace) measurements made of a sample of the FIG 3 VECSEL. Figure 6 is a schematic diagram of another optical fiber transmitter unit including an optically pumped MQW VECSEL. Figure 7 is a schematic diagram of a MQW VECSEL having an annular piezoelectric element for providing micro adjustment of the external cavity length. Figure 8 is an enlarged schematic diagram of a MQW VECSEL in which a 150 mW DFB diode laser is mounted and aligned to illuminate the VECSEL semiconductor structure at Brewster's angle. Figure 9 is an enlarged schematic diagram of a MQW VECSEL illustrating an arrangement for aligning and securing the external mirror spacer to the heat sink to achieve precise cavity length. Figure 10 is an enlarged schematic diagram of a MQW VECSEL including an intra-cavity etalon enabling rapid mode selection. Figure 11 schematically illustrates an embodiment of a VECSEL having a. resonator incorporating an amplifying mirror and an electronically adjustable tuning element within the laser cavity. Figure 12 shows spectra of a resonator, amplifying mirror gain and tunable element wherein the gain is flatter than that shown in FIG 1. Figure 13 shows an amplifying mirror incorporating a Bragg reflector region and a gain region within a monolithic structure. Figure 14 illustrates an amplifying mirror excited optically by a laser beam originating outside the cavity. Figure 15 illustrates an electronically adjustable tuning element having a tilted plane etalon with an electro-optically active spacer and a polarization selective element. Figure 16 illustrates an electronically adjustable tuning element having a Brewster-angled window as a polarization selective element and a birefringent electro-optical medium to select the desired lasing mode. ^Figure 17 illustrates the effect of a tilted birefringent tuning element upon the polarization of the light in the laser cavity. Figure 18 illustrates an electronically adjustable tuning element comprising a plurality of electro-optically active birefringent media. Figure 19 illustrates an electronically adjustable tuning element a plurality of electro-optically active birefringent media together with additional birefringent media without electro-optical control. Figure 20 illustrates the polarization altering role of the birefringent, but non- electro-optically control, media in the embodiment of the device shown in FIG. 19. Figure 21 illustrates an electronically tunable frequency selective element incorporating a lens to create a beam waist within a tuning device in addition to the beam waist present in an amplifying mirror. Figure 22 illustrates an electronically tunable frequency selective element incorporating an off-axis concave mirror to create a beam waist within a tuning device. Detailed Description of the Drawings The present invention can provide a compact, tunable laser which preferably can be tuned over a range of several tens of nanometers within an optical telecommunications multi-channel band and which will operate reliably and for extended periods on selected channels thereof (for example as specified by the International Telecommunications Union (ITU) DWDM grid plan). The laser source may be an active mirror (MQW VECSEL or SOA) having an epitaxially grown or vapour deposited or otherwise realized MQW gain structure with a reflector (e.g. an incorporated distributed Bragg reflector (DBR)) or a dielectric coating and an external dielectric coated spherical or other mirror. At least the first mentioned mirror preferably has a reflectance of at least 90 percent. An intra cavity tuning element can be used to achieve rapid frequency selection. The external mirror is positioned to give an optical path length L from the MQW structure such that c/2L = Δv DWDM , where c is the velocity of light and Δv D DM is a required telecom optical channel spacing, for example 12.5 GHz or 25 GHz. For a channel spacing of 25 GHz the effective optical cavity length L should be approximately 0.6 cm. L may include thickness and dispersion compensation for any intra cavity elements. Separation between the MQW structure and the external mirror may be maintained by a spacer made, for example, of a material having a low index of thefmaP expansion, , such as fused silica (α ~ 10 "6 ) or Zerodur tm , a silica-like material made by Heraeus-Amersil and which can have a thermal expansion coefficient equal to essentially zero over a temperature range of several tens of degrees C. Alternatively, if the cavity components can be mounted on a miniature optical bench having a low (or controllable) index of thermal expansion, and then a free space version of the laser is also possible. The resultant optical cavity provides a comb of fixed lasing frequencies. An absolute frequency reproducibility Δv/v = ΔL/L = αΔT of 6.3 x 10 "6 (for T = 6°C) can be realized with a fused silica spacer. In this laser, the external cavity itself serves as an absolute frequency standard (etalon). The spacer length tolerance δL should preferably be L x 6.3 x 10 "6 = 0.04 μm, or 0.025 of the working wavelength. Such accuracy is achievable by known methods, and retardation plates can be made to adjust a polarization vector with better accuracy than one degree of retardation. The radius of curvature of the external mirror depends on the cavity mode diameter at the MQW structure. For example, with external mirror spacing L = 0.6 cm a cavity mode diameter of 50 μm will be achieved if the mirror radius of curvature is equal to 0.63 cm. Such mirrors can be manufactured by known methods, such as molding against a diamond-turned metal preform. In all laser designs, the cavity must be a stable resonator. Other devices may be used, such as plano/plano mirrors used in conjunction with an intra cavity lens. Preferably, single frequency operation on a particular telecom channel is achieved: by the homogeneous broadening properties of the MQW gain structure which result in spectral narrowing of laser output radiation after its initial buildup within the cavity, and by laser cavity design in which the active gain structure is positioned so as to minimize spatial hole burning effects and to favor single frequency operation, even if no additional frequency selection mechanisms (such as intra-cavity etalons, vertical coupler or Lyot filters) are provided. The transient behavior of certain VECSELs has been reported in the articles by co-inventors Garnache et.al. cited above. For a VECSEL having a homogeneously broadened gain medium with gain bandwidth T (half-width at half-maximum (HWHM)) and broadband mirrors, the intensity M q (t g ) of a mode q at a time t g (generation time) which is measured from the instant when pumping started can be described by the following equation (1): where q 0 is a central mode number, and γ is the cavity loss rate. Cavity loss rate γ can be described by equation (2): — clnli2 0C (l-/ / ) 2 J/2 , (2) where c is the velocity of light, and the cavity has an output coupler having reflectivity R^, and internal loss 1,-. From equation (1) it follows that after a VECSEL starts lasing, its spectrum will be multimode with the total width close to the gain bandwidth 2r, but the intensities of the side modes will decrease exponentially over time so that the spectral width Δv (HWHM) will decrease in time inversely proportional to the generation time t g , in accordance with equation (3): _ v = T^(2)l)f g . (3) It has been experimentally confirmed that a requirement for validity of equation (1) is that the gain medium of the active region of the VECSEL is homogeneously broadened and that any non-linear interactions between the modes are negligible for a given generation time. If, for a certain laser, the spectral width becomes smaller than cavity mode spacing, and equation (1) remains valid, the laser will collapse to single frequency operation. In the September 2000 JOSA-B paper of co-inventors Garnache, Kachanov and Romanini with co-author Houdre, it was shown that for a VECSEL active region comprising a strained InGaAs MQW in GaAs, this equation is valid at least for a generation time. t g as large as one second. If one assumes a reasonable .value of gain bandwidth T as equal to 100 cm "1 or 3000 GHz, external coupler reflectivity Roc = 0.99 percent, and cavity internal loss lj of 0.001, then for a cavity length L of 0.6 cm, the spectrum must collapse to a bandwidth smaller than intermode spacing equal to 25 GHz at a time t g ~ 0.03 msec. This means that the VECSEL will then be lasing at a single frequency in a mode closest to the gain maximum. This time is significantly shorter than the time for which equation (1) was experimentally confirmed as reported in the above-referenced article. Note that for a percent output coupling which corresponds to an even faster spectral collapse time of 3 μsec. In principle, actual laser wavelength switching should occur in no more than 1 μsec. FIG. 1 presents two plots of VECSEL active gain as a function of wavelength and pumping intensity, and marks a series of VECSEL cavity modes (resonances) across the abscissa. The dotted line shows initial intensity buildup in the cavity during startup (of particular importance in the case of ICLAS). After about one microsecond the VECSEL active gain becomes clamped to the average cavity losses. From the solid-line curve of FIG 1 it can be seen that only the mode closest to the gain maximum will be lasing. By adjusting pumping radiation to an appropriate level, and by thermally decoupling the VECSEL external cavity mirror, it is practical, by thermal control, to move the VECSEL laser output wavelength slowly and stably from mode to mode over time. See FIG. 2. Faster mode selection (<1 μsec) may be achieved by providing a controllable intra-cavity element such as an etalon or Lyot filter. Thermally controlled VECSEL mode hopping and stability at each selected mode (following startup phase) over hours is shown in FIG. 2 which relates to a VECSEL having a semiconductor structure grown by molecular beam epitaxy on a 0.5 min GaAs substrate. The bottom stack of the VECSEL was a standard Bragg mirror and consisted of-30.5 pairs of AlAs-Alo .27 Gao.g 3 As quarter-wave thick layers with a measured reflectance of 99.96 percent at a 1030 nm laser design wavelength. The active region (MQW) consisted of two sets of three strained 8nm In 0 .2Gao. 8 As quantum wells separated by lOnm GaAs barriers. Each set of quantum wells was placed at the maximum of the intracavity standing wave. 830 nm optical pump radiation was focused into the GaAs absorbing layers within the gain region (which had an optical thickness of 3λ/2. An AlAs quarter-wave layer followed by an Alo . o- 7 Gao. 23 As half-wave layer was grown on top of the active region to prevent carriers from diffusing into the semiconductor surface and to provide an Al-poor surface to avoid surface 1 contamination: A Ti:sapphire pump laser emitting 830 nm pump radiation was' focused into the gallium arsenide absorbing layers within the gain region. The semiconductor chip was soldered onto a copper heat sink and cooled by a -Peltier element and a heat radiator operably connected to a temperature controller. The optical pump source, the VECSEL structure and the external cavity mirror were mounted onto an aluminum base plate. Tunability was achieved over a range of 1012 to 1086 nm with less than 500 mW of pump power. See Optics Letters, Vol. 24, No. 12, June 15, 1999, pp 826-828, cited above. As shown in FIG. 1, after a very short generation time (t g ~ 0.03 msec, for example) the telecommunications VECSEL will operate at a single frequency in a mode closest to the MQW gain peak. The gain (solid line) will be clamped to the cavity loss value (horizontal dashed line) at this operating mode frequency (clamp is denoted by small full circle). If the temperature of the MQW active region is changed, for example by changing the set point of a thermoelectric cooler (element 112 in FIG. 3) of the semiconductor structure, the gain maximum will move to a higher or lower frequency, corresponding to an adjacent channel. The VECSEL will remain in the same mode until the gain maximum reaches a half-distance between the two adjacent modes. At this point due to spontaneous emissions or mechanical or other ever-present perturbations, the laser will intermittently mode hop. As the gain curve is thermally moved further, the laser will start lasing single mode on arrival at a mode adjacent to the departure mode. This experimental behavior of a non-stabilized VECSEL having a cavity length of 2.5 cm and with an InGaAs active region is shown in FIG. 2. FIG. 2 shows a very stable single mode operation with very low frequency drift approximating 6 MHz per minute, even though the laser case temperature was not stabilized, and the external mirror spacer was provided by an aluminum base plate. FIG 2 shows zones of intermittent behavior when the gain maximum is located equidistantly between two adjacent modes. Because the cavity design of the present VECSEL preferably enables operation orily ' on a particular channel, temperature tuning provides a very efficient and stable way to tune from one channel to another. The tuning range of this VECSEL depends on the band gap energy dependence of the active zone materials. The teiriperatufe tuning coefficient of the laser described in the September 2000 JOSA-B paper referred to above was 0.45nm/°C. By changing the temperature set point to-'the range of + 25°C with respect to room temperature, a tuning range of approximately 23 nm can be realized. The spectrum of the laser will reach a stationary state at some time τ sp , which is determined by the spontaneous emission rate into the cavity modes, and it can be found from Equation (4): τ _ X __ (4) rξ where Mo st is the intensity (photon number) of the central mode, and ξ is the ratio of the spontaneous emision rate into one laser mode to the stimulated emision rate per photon, (which is close to unity). The spectrum of the laser in this stationary state will have a Gaussian shape with a width, which can be evaluated from equation (1). If it is assumed that the power output of the VECSEL is 10 mW, then the photon number in the central mode will be 3 x 10 8 photons, τ sp will then be 1.7 seconds, the width of the Gaussian spectral distribution will be 0J7GHz, and relative intensity of the nearest neighbor to the lasing mode will be 4.7 x 10 "5 , or -43 dB. A fiber optic telecommunications transmitter 100 is schematically illustrated in FIG. 3 for putting modulated optical power emitted by a VECSEL 104 into an optical fiber 106 of a communications network operating in the near infrared spectrum, e.g., 1000 nm to 1700 nm and having a multiplicity of spaced apart channels therein, e.g. 12.5 GHz or 25 GHz adjacent channel spacing. A suitable amplitude modulator 105, such as a lithium niobate or lithium tantalate crystal, or other electro-optical element or grating having a current-modulated index of refraction or an electroabsorption modulator, for example, is preferably included in the radiation path of the VECSEL 104 in order to impart the necessary information signal to the VECSEL laser beam before it passes into the optical fiber 106. A laser diode unit 102 puts out optical pumping radiation at a desired wavelength, e.g. 980- 1000 nm (1.24 eV at 300K), and power level, e.g. a minimum of 150 mW and typically 250 mW to 1 W. When excited by e.g. 150 mW of pump power, the VECSEL 104 puts out 5 mW in the near infrared spectrum, e.g. 1560 nm, and output power scales up as a function of pump power. A folding mirror 103 may be provided to direct the pump radiation toward the VECSEL 104 which has a relatively c short cavity, e.g. on the order of 6 mm, between an intrinsic DBR mirror (see 126 in FIG 4) of a semiconductor structure 114 and an external cavity mirror 116. Other elements of the VECSEL 104 include a heat sink 110, a thermoelectric cooler 1*12 for wavelength control (mode selection), and an epitaxially grown inverse semiconductor structure- 114 including an antireflection layer, the MQW active gain region, the DBR mirror layers and a metal film mirror layer adjacent a heat sink (base plate) (see layers 122, 124, 126, 128 and 110 respectively in FIG. 4). A spacer 115 supports an external mirror 116 over the top surface of the semiconductor 114 at a precise distance establishing the VECSEL cavity spacing in a range preferably between 0.5 mm and 50 mm, (approximately 6 mm in the present example). While the spacer shown in FIG 3 is cylindrical, it may have a variety of geometric or other shapes and be comprised of a single integral element, or several integral or discrete elements such as posts, pillars, troughs, or elements of any other shape desired for a particular application. Thus, the term "spacer" as used herein includes multiple structural elements as well as single integrated structures. The spacer 115 can precisely fix a VECSEL cavity mode separation to be equal to a dense wavelength division multiplex ("DWDM") channel spacing of optical telecommunications network. The ITU has established the DWDM telecommunications band at 190 THz, with 25 GHz channel spacing. This requires an accuracy of absolute mode positions equal to 2.5 GHz. In order to provide absolute frequency control of each mode equal to ten percent of the nominal channel spacing, a VECSEL cavity length precision on the order of 1.25 X 10 "5 ( ΔE / L) is required. The spacer 115 is preferably formed of a material, such as a molded glass component, (e.g. ULE glass, quartz, Zerodur ™ or other glass or metal such as Invar ™ having a low coefficient of thermal expansion) which thermally decouples external mirror 116 from semiconductor structure 114. Thus, temperature regulation of the active region 124 (see FIG. 4) with, for example, thermoelectric cooler 112 and heat sink 110 does not changeJhe length of the VECSEL cavity. The spacer material can compensate for changes in the length of the semiconductor structure 114 when the structure is heated. For example, if the semiconductor structure elongates with temperature, the spacer material is chosen to expand with temperature by an amount to offset any change in wavelength, which-would otherwise result. With a spacer made of fused silica having a thermal expansion coefficient α of 10 "6 , channel separation will be maintained within a temperature change of plus or minus ten degrees C. Spacer materials such as Zerodur ensure mode positions within temperature changes of plus or minus 100 degrees C. The VECSEL absolute cavity length during manufacture should preferably be reproducible within 0.4 micron. Thickness tolerances of optical materials within 0.5 micron can be achieved by known methods. Trimming of the cavity length under optical feedback control may be used to meet the required cavity length tolerance. The external mirror 116 may be a separate element bonded or otherwise located at or onto the end of the spacer 115, or ' it may be formed integrally with the spacer. The mirror 116 may be molded to the desired radius with the aid of a diamond turned metal preform or shaped to a desired spherical contour by any other known method. The mirror surface of structure 116 is of a very high reflectance, and it has a desired spherical radius of curvature relative to the MQW active gain region to define a first highly reflective concave surface. The mirror 116 and the semiconductor structure 104 form a VECSEL cavity having sufficiently high finesse to realize effective single mode lasing operation when excited by an appropriate level of optical pumping energy. The interior environment of the VECSEL 104 may be dry air, helium, nitrogen, vacuum, or another medium, depending upon acceptable scattering/absorption tolerances. The external mirror 116 may have an outer radius of curvature so as to form a coupling lens for focusing the pumping energy into the active region 124 and/or for focusing the VECSEL laser emission beam into the optical fiber 106. Losses in the laser cavity of VECSEL 104 need to be low enough so that the gain of the MQW active region is sufficient to overcome those losses. An external mechanical/optical coupler 108 may be provided to position a fiber end and couple the VECSEL laser beam into the fiber 106. Other known laser/fiber coupling arrangements may also or alternatively be employed to position and stabilize the components and/or to couple the beam into the fiber. The transmitter also preferably includes a beam splitter 117 in the output path, which directs a component of output radiation into a photodetector 119. The photodetector provides optical feedback information in the form of electrical signals to a controller 121, preferably a programmed digital controller. Controller 121 generates control 1 signals which are converted into driving currents by an amplifier 123 and applied to control the thermoelectric cooler 112. Performance of transmitter 100 may be improved by including a temperature sensor 125 within the body of cooler 112 and by feeding sensed temperature values back to controller 121. The controller can also feed back pump laser current control values to the pump laser to control pump radiation to maintain a constant output power out of the VECSEL via higher pump power levels at the edges of the tuning range where VECSEL gain is not as high. Controller 121 includes known analog to digital and digital to analog conversion circuits. Even though the transmission of the DBR mirror 126 (FIG. 4) of the semiconductor structure is very low, a small amount of the transmitted light reflected from the back side of the substrate can reenter the cavity. This small amount of light can introduce a spectral perturbation which has the form of fringes with the spectral period defined by the optical thickness of the substrate. Such modulation is unwanted and may impede smooth tuning from one mode to another. This effect can be significantly reduced by providing a wedge-angle of several degrees to the substrate. However, for telecom applications, such wedge shape may not be sufficient to completely remove this effect because the VECSEL will operate in a stationary state and the influence of the light scattered from the substrate may prove to be strong enough to perturb smooth tuning from mode to mode. In order to exclude completely any optical perturbation from the back side of the substrate, we preferably deposit a non-transparent metal film layer between the DBR and the substrate, especially by a reverse order epitaxial process. The semiconductor structure 114 of FIGs 3 and 4 is grown in reverse order on buffer layer 120 most preferably by molecular beam epitaxy: AR layer 122, then active region 124, then DBR mirror 126 and finally metal film layer 128. The VECSEL semiconductor structure 114 is preferably grown on a substrate 118 comprising InP having an etch stop layer 120 of In x Ga^ x As, where x and 1-x represent chemical mole fractions of the respective elements of the crystalline buffer layer material. As grown in reverse order, the first or bottom layer comprises the antireflection layer 122 of a λ/4 thick indium phosphide capping layer and an indium gallium aluminum arsenide layer of a thickness 5λ/4 where lambda represents the VECSEL nominal mid-band output wavelength, 1560 nm, for example. The function of the antireflection layer 122 is to prevent reflection within the semiconductor structure at the VECSEL lasing wavelength, as opposed to the pumping wavelength. Because the VECSEL 104 is pumped through the external mirror 116 or through some other lens or opening through spacer 115, the antireflection layer 122 should be of a material selected for minimal absorption of energy at the pump wavelength, so that a maximum pump power will enter the active region, excite the quantum well carriers and yield efficient lasing. For telecommunications it is necessary that the antireflection coating 122 be effective across the entire optical communications band, and not just for a single channel. A positive gain, active region 124, in one example having a length 1X14, is then formed and has, for example, three pairs of quantum wells 130 of indium gallium arsenide. While FIG. 4 illustrates an arrangement of pairs of quantum wells 130 with each pair arranged at a peak of the optical standing wave, other arrangements can be employed when optical pumping radiation enters the active gain region 124 via the antireflection coating 122, such as three-two-one. In this alternative arrangement three quantum wells are at an optical peak nearest antireflection coating 122, two are at a middle peak, and one is at a peak distant from the antireflection coating. In the near infrared spectrum, quantum wells typically have less gain than in the visible . spectrum, and a sufficient number of quantum wells must be provided to yield the needed gain for reliable operation at the desired output power. Each quantum well 130 has a thickness designed in relation to the desired output wavelength at the operating temperature (which, because of absorption of the pump energy, will be higher than room or other ambient temperature). When the active region 124 is pumped, it heats up. When a semiconductor is heated, it changes effective thickness and index of refraction. Accordingly, the emitting wavelength of the quantum wells 130 must be shifted to a higher energy level compared to a room-temperature design wavelength of the structure: λow ~ λ DE si GN - 20/30 nm approximately (at T 0 = 300 K, at low excitation), so that the gain and the design wavelength match when the VECSEL 104 is lasing. In FIG. 4, each pair of quantum wells 130 is located at a maximum of the active region standing wave. Major separation or barrier layers between the quantum well pairs have a length optimized for an absorption coefficient at the pump radiation wavelength which in this example is 980 nm. FIG. 4 not only shows a diagrammatic cross section of the layers of the semiconductor structure 114, it also plots relative band gap energies of the various semiconductor layers 120, 122, 124, and 126. A distributed Bragg reflector (DBR) layer stack 126 is then formed on top of the active region. The DBR 126 comprises an odd number of quarter-wavelength interleaved layers, preferably greater than twenty pairs plus one, to achieve an odd number of quarter wavelengths. The DBR layers comprise alternating indium gallium aluminum arsenide, and indium phosphide quarter wave layers, so that total reflectance within the DBR at the design wavelength is greater than 99 percent. Finally, a metal film mirror layer 128, e.g. gold or gold alloy, is sputter-deposited onto the DBR 126 to complete the fabrication of the semiconductor structure by the epitaxial process. The metal mirror increases the reflectance from 99 percent to approximately 99.5 percent. Index matching and phase discontinuity issues are essentially avoided by using both the DBR 126 and the metal mirror 128. The effective length between the DBR 126 and the external surface of the antireflection coating 122 is set to an odd number of nominal output quarter wavelengths, so the sub-cavity formed by the semiconductor structure 114 operates in anti-resonance. The pumping energy passes through an antireflection coating 122 to reach an active region 124 of quantum wells. For a fundamental transverse electromagnetic mode (TEMoo) operating wavelength of 1560 nm in the near infrared spectrum and at the design operating temperature (300 K), the various layers of a semiconductor structure 114 between the gold layer 128 and a substrate layer 118 are given in the following table. FIG 5 is a graph showing an upper sinuous line representing measured reflectivity of a sample semiconductor structure 114 in accordance with the above table, and a lower sinuous line representing measured photo luminescence of the same sample. After the epitaxial deposition processes are complete, the indium phosphide substrate 118 is removed by abrasion, such as ion beam milling. The indium gallium arsenide etch stop layer 120 is then removed by a second step of wet chemical etching with an etching agent which favors removal of the gallium arsenide substrate 120 (rather than the indium phosphide capping layer within the anti-reflection layer 122). As indicated in the table above and in FIG. 4, a dielectric layer of appropriate thickness is then vacuum deposited onto the InP capping layer in order to complete the e.g. 5 1/4 thick antireflection layer 122. This gives rise to proper P polarization as with a λll plate, such that the ellipticity turns into a circular pattern at the surface. A wafer including multiple semiconductor structures 114 is then diced to yield individual semiconductor dies or "chips". The metal mirror layer of chip 114 may then be bonded to a silicon substrate or soldered to a copper heat sink 110. By removing substrate 120 in this preferred reverse order process, thermal control of active region 124 via thermoelectric cooler 112 and heat sink 110 is much more direct and positive, than if heat had to be conducted through the substrate, as is the case with conventional VECSEL and VCSEL designs. For example, for telecom applications, a thermal control loop comprising elements 117, 119, 121, 123, 112 and 125 shown in FIG. 3 is preferably employed to establish and maintain a central or reference wavelength within a multi-channel optical band. As shown in FIG. 1, the VECSEL 100 will lase on a mode closest to the gain maximum. When the temperature of the quantum well active region 124 is changed, the gain maximum shifts with a tuning of approximately 30 GHz/°C. This tuning range indicates that if the temperature of the gain structure is kept stable within 0J°C, the VECSEL will lase on a single selected mode. This approach avoids the drawbacks of absolute temperature control required for existing telecommunications DFB lasers. As shown in FIG. 2, when the gain maximum is thermally tuned to a position corresponding to the middle between two adjacent modes, VECSEL 104 will intermittently lase on one or the other mode. If the laser is alternating between two modes, such alternation can be detected by providing photodiode assembly 119 with frequency selective-filtering characteristics. By thermally tuning VECSEL 104 over the entire frequency range, it is practical to detect all set points corresponding to gain positions exactly between the modes. These positions are then stored in control unit 121 enabling an exact match between temperature set points and the DWDM channel to be computed and presented to active gain region 124 via cooler 112. This particular method enables thermal compensation over time for changes in the semiconductor structure materials of VECSEL 104 due to material aging, for example. Control unit 121 also controls a startup sequence of pump laser 102 and regulates pump power during steady state operation, so that VECSEL 104 is started single mode and remains so. The intermittent, mode hopping behavior shown in FIG. 2 is most preferably used to determine automatically the relationship between temperature set points and telecom channel numbers. The temperature set points should correspond to the situation where the gain curve maximum coincides exactly with the corresponding mode, as illustrated in FIG. 1. In FIG. 2 the intenmttent laser behavior takes place when the gain maximum is located exactly between two modes, and the temperature range corresponding to such intermittent behavior is significantly smaller than the temperature increment necessary to move the laser from one mode to its nearest neighbor. Thus, these temperature set points can be determined with a very high precision by observing the laser output with monitor photodiode 119 which has in front of it a filter having an optical transmission gradient which varies from a low of a few percent, to a high near 100 percent, across the telecom wavelength range. With such a detector, which is AC coupled to control unit 121, jumping of the laser from mode to mode will result in intense random spikes (mode transitions) which produce digitized values and intervals readily detected by control unit 121. A presently preferred method for laser self-calibration is as follows. Control unit 121 sets the laser temperature set point to its lowest negative value causing cooler 112 to reach its lowest nominal operating temperature. Then, control unit 121 causes the temperature to increase over a calibration time interval. Control unit 121 continuously monitors laser output to detect mode transitions, and it records the temperature control parameter for that particular transition in memory, most preferably a non- volatile electrically rewriteable memory within control 121, such as a "flash" memory element or array. This operation of detecting mode transitions and recording thermal control parameters continues until a maximum positive temperature set point is reached. The control value for the first mode is then one half the first thermal control parameter, the control value for the next mode is then the median between the second and third recorded control parameters, and so forth, until all control values within the thermal control range are determined. This dependence can be expressed as a low order polynomial such as U t (n), where U t is, for example, the control voltage input to cooler 112. Then, the control value of the temperature control (corresponding e.g. to voltage or current) for the first telecom channel accessible by this particular laser will be found as U t (l), the control value for the second telecom channel will be U t (2), etc., and finally the control value for the last telecom channel accessible by this particular laser will be found as U t (n). Obviously, U t (0.5), U t (1.5), etc., are the set points corresponding to the gain curve equidistant between the adjacent modes. Finally, the absolute wavelength of the first operating channel for the particular laser may be determined and preferably recorded in the controller memory (and preferably also in documentation accompanying the laser) at the factory by use of an optical spectrum analyzer, such as a Burleigh WA-7100. Once calibrated, the laser will keep the values for a long time. The distance in wavelength units between two telecom channels with 25 GHz channel spacing is 0J95nm. Provided that the laser temperature tuning coefficient is about 0.45nm/°C, the tuning from one telecom channel to another will require about 0.4 °C temperature increment. In order to keep the laser operating at the chosen channel frequency, a readily obtainable temperature tolerance of 0J °C is sufficient. After the laser leaves the factory, the automatic calibration of the laser can be repeated in the field or other operating environment at any time, and any correction to temperature control values (polynominal coefficients) can be refreshed by control unit 121 and replaced in memory. A spectrum analyzer will not be required unless initial laser calibration data is lost. Calibration may thus be maintained for close to the useful life of the laser. VECSEL 104 is presently preferred because of its simplicity, but other arrangements may be employed. In FIG. 6, a VECSEL-based telecommunications transmitter 200 includes a laser diode pump 202, a pump beam focus lens 203, and a VECSEL 204. In this configuration, pump energy enters one side of VECSEL 204, and laser energy exits from another side. A base 210, made of thermally conductive material such as copper, defines a pump aperture 211. A thermoelectric cooler such as cooler 112 (not shown in FIG. 6) automatically controls temperature of the base 210. A semiconductor structure, formed in reverse order in the same manner as the FIG. 3 VECSEL 104, has a pump-transparent dielectric mirror layer 214, an active gain region 216 and an antireflection coating 218. An external surface of the dielectric mirror layer 214 is polished very smooth and is bonded to a transparent substrate layer 212, such as diamond, by a suitable bonding method or agent. Vacuum bonding by VanDerWaals forces, or peripheral soldering with a solder material, such as indium, is preferred. External lens 220 and spacer 222 are equivalent to those of VECSEL 104, and the explanations given above apply here. With suitable modifications that will be apparent from by the foregoing explanations, the pump laser radiation could enter the active region of VECSEL 204 via external mirror 220, and the laser radiation could exit via aperture 211. When single wavelength operation and discrete step-tuning is not necessary, such as in spectroscopy, the spacer, instead of being formed of low thermal expansion materials may be made of a material that changes its dimension under external stimulation, such as a piezo-electric transducer, or as a result of changes in external ambient pressure. FIG. 7 shows a MWQ VECSEL 300 in accordance with these principles. VECSEL 300 is similar in structure to VECSEL 104 and common elements bear the same reference numerals and the previous descriptions apply. However, in VECSEL 300 an annular piezoelectric element 302 is sandwiched between heatsink 112 and spacer 115, and around base 110. In this case the VECSEL cavity length can be made even shorter, e.g. 1 mm or even a few hundred microns, and the laser will provide single-frequency mode-hop tuning over several wave numbers (cm "1 ). In this regard it is important that the piezoelectric element 302 apply a uniform force around its circumference, so that external mirror 116 remains on the optical axis of VECSEL 300 as the cavity is lengthened or shortened. Alternatively, if the external mirror is coupled to a piezoelectric transducer, the VECSEL will work as a tunable single frequency source. The mode hop free tuning range will be close to the cavity mode spacing, (25 GHz for a cavity length of 0.6 cm.) If the cavity length is reduced to about 1 mm for example, the mode hop free tuning range (without synchronous MQW structure temperature tuning) will be 150 GHz, or 5 cm "1 , which makes the resultant VECSEL a very good source for spectroscopic and gas analysis applications. Use of a conventional laser diode pump can reduce cost. Optical pump radiation is directed at the surface of the semiconductor active region at an angle of incidence equal to Brewster's angle (θ) and with P polarization which maximizes absorption of pump radiation into the semiconductor structure 114 and minimizes reflection at the surface thereof. See FIG. 8 which shows a MWQ VECSEL 400 in accordance with these principles. VECSEL 400 is similar in structure to VECSEL 104. In FIG. 8, a miniature diode laser assembly 402 is aligned and secured within an opening of spacer 115 of VECSEL 400 so that pump radiation is directed with P polarization at the external surface of semiconductor structure 104 at an angle equivalent to Brewster's angle (θ), about 73.6 degrees. A micro-lens 404 may be included within the DFB laser assembly 402 to collimate the pump beam and limit spot size to between 50 and 70 microns, for example. Directing a pump beam from a diode laser having a three to one ellipticity at an angle of incidence equivalent to Brewster's angle and made to have proper P polarization as with a λ 1 plate, results in ellipticity turning into circularity at the surface, and in minimization of mode mismatches between the pump laser beam and the semiconductor. In this manner the diode laser assembly 402 need only emit at a power level of 150 mW to obtain an equivalent pump effect to that of a much higher power laser pump having a beam not incident at Brewster's angle. 150 mW diode lasers emitting at 980 nm wavelengths are available, from Nortel, JDS and Coset. The pump assembly 402 may have an integral heat sink and thermoelectric cooler in order to facilitate thermal control of wavelength in addition to direct current control. The polarization of a conventional lx3μ high brightness laser diode is oriented parallel to its long emitter side, so as to be opposite to what is wanted in order to pump at large angles of incidence, i.e. to expand the short diameter due to the large angle. In this case a λ 12 plate will be required in order to rotate the polarization. Such diodes have some astigmatism, that is to say, the virtual point sources which represent the diode radiation have different positions along the diode's Z axis for fast (x) and slow (y) laser diode convergence planes. When imaged with a lens, the diode surface image therefore has different locations for x and y planes. The diode can therefore be oriented so that its polarization is a right P polarization (3 μ side in the incidence plane) and some intermediate plane used between the two astigmatic image planes in order that the spot on the VECSEL semiconductor structure surface be circular. The arrangement shown in FIG. 8 is presently preferred as it enables the pump laser 402 to be fixed in place in the factory during VECSEL assembly, and then checked before delivery. If a small portion of pump energy is reflected by the outer surface of the semiconductor 104, an aperture or other pump-energy absorbing means 406 may be provided in an opposite position in the spacer 115 to prevent or impede further reflection of pump energy within the external cavity. Spacers can be manufactured within a 1 μm tolerance. During manufacturing it is necessary to make a final adjustment of spacer length so that it meets the required 0.04 μm tolerance in order that laser mode spacing equal the telecom channel spacing. This fine adjustment is also required in order that the absolute frequency of any given mode be within the required tolerance equal to the absolute frequency of the telecom channel closest to this mode. It does not matter which mode is tuned to the nearest telecom channel providing that the mode spacing corresponds correctly with the telecom channel comb. The channel number can be adjusted in the processor unit 121. To perform fine adjustment of the spacer, the VECSEL should be switched on so that it lases single frequency at some mode. Its output radiation is sent to a spectrum analyzer, such as a Burleigh WA-7100 spectrum analyzer, which provides a wavelength measurement accuracy of ± 1.5 ppm or + 0J9 GHz. The analyzer will display the lasing frequency. In FIG. 9, mirror 116 is fixed to spacer 115 such that the border of its spherical surface abuts the polished flat surface of the cylindrical spacer. Mirror 116 is located in a spring loaded mount or other fixture, so that by adjustment of, for example, a spring, the spacer can be axially displaced relative to a base plate 502, or can be given a small elastic deformation. Base plate 502 may be provided with a recessed or flanged region 504 sized to provide a small interference fit with spacer 115 so that the spacer is initially maintained at a starting position. An adjustment of 1 μ along the longitudinal axis of VECSEL 500 can readily be made while monitoring a spectrum analyzer. When the spacer has been adjusted to give the external cavity its precisely correct length, it is securely bonded to the base plate by a bonding agent 506, such as low temperature glass, solder, UV-curable resin system, or the like, (which may be heated and flowed or reflowed before this adjustment). Final fixing may be achieved by post-tensioning the spacer 115 with three or more tensile posts or other members 510 which are automatically tensioned by computer control in the factory between a flange 508 of the spacer and fine-pitched threaded openings formed in base plate 112. Other arrangements, such as maintaining a spring bias force on the spacer, may be employed but usually with slightly greater mechanical complexity. An additional electro-optical control element, such as a thin dielectric tilted etalon having dielectric partial reflective coatings, for example, can be included within the VECSEL cavity. Such an etalon will work as a bandpass filter, and will reduce effective gain bandwidth, thus reducing the time necessary to reach single frequency operation and increasing side mode suppression. The wavelength of the etalon transmission peak should be close to the gain maximum. The tuning of the etalon may be achieved by, for example, changing its temperature using a separate thermoelectric or other cooler element, or by changing its tilt angle with a piezoelectric or other control element, or by changing the index of its spacer material electro-optically. Correspondence between etalon temperature/tilt angle and a selected cham el can be established using a procedure similar to the calibration procedure set out above. Such an additional element or controllable wavelength filter enables more rapid selection/control of VECSEL emission wavelength than may be realized by mere thermal control. In addition, the intra-cavity element may be provided to speed up single mode operation and to reduce the possibility of multimode lase during startup. The closest cavity mode to the transmission peak of this element becomes the chosen operational mode. As the DWDM channel separation is 10 " of the telecom channel frequency, positioning of the intra-cavity element's optical peak has to be made with an accuracy of a few percent of the free spectral range. Rapid tuning to a particular channel can then be carried out by the intra-cavity element, which effectively changes the cavity length in a controlled manner correlated to the selected channel. From 500 to 1000 separate channels can be realized over a 200 nm wavelength tuning range centered at e.g. 1560 nm. FIG. 10 is an enlarged schematic diagram of a VECSEL 600 including an intra-cavity element 602 forming a mirror for reflecting optical energy emitted by the semiconductor structure 114 to a spherical mirror 604. A two-part spacer includes a generally cylindrical body 606 which defines the mirror 604, and further includes a plate 608 which aligns and secures the intra-cavity element 602. The etalon 602 may be partially transmissive, and the monitor photodiode 119 then can be mounted to the plate 608 behind the etalon 602. The controller 121 includes an etalon driver 610 for driving either a thermoelectric cooler or a piezoelectric transducer which controls the etalon 602 as described above. The STECAML may comprise an active mirror, with a gain bandwidth sufficient to overlap several modes of the cavity formed between that mirror and another external mirror, and some means of selecting the cavity mode that oscillates and of altering that selection. That selecting means is preferably a tuning element placed inside the cavity. The amplifying mirror may be fabricated as a surface emitting laser gain structure. Alternatively, the amplifying mirror may be fabricated as an edge-emitting semiconductor optical amplifier (SOA). Frequency-switched radiation for optical communications or for spectroscopy may therefore be provided by shifting the transmission band of a wavelength dependent intra-cavity mode selector in a STECAML cavity by means of an externally applied electrical signal. The STECAML cavity is designed to have fundamental axial modes at pre-specified wavelengths. The mode selector determines which mode lases at a given time, rather than tunes to a wavelength about some pre- specified value. The use of electro-optic media such as lithium niobate, lithium tantalate and smectic and nematic liquid crystals in the tuning means allows the radiation wavelength to switch on within a millisecond or less, which is much more rapid than can be achieved by mechanical tuning. Furthermore, no radiation is produced at unwanted wavelengths. In the embodiments of FIGs. 11-20, the gain spectrum of the amplifying mirror is less sharply peaked than in FIG. 1. The layer designs of the MQW and DBR structures are only slightly different from those illustrated in FIG. 4 and FIG. 5. When the gain spectrum is relatively flat, there is a need for an intra-cavity element to select one cavity mode for oscillation. However, once the power in that mode increases, saturation suppresses the gain of the other modes as shown in FIG. 1. In FIG. 11 amplifying mirror 20 and concave mirror 25 constitute the laser cavity of a VECSEL. The output beam is shown schematically as 60. A heat-sink device 11 is outside the cavity but forms part of a mechanical support structure. An electronically adjustable frequency selective element 30 responds to externally- originated electrical signals, sent on cable 40, by switching the laser output 60 from one frequency to another. FIG. 12 illustrates the spectral output of the VECSEL of FIG. 11. The cavity formed by mirrors 20 and 25 produces a spectrum of modes 70 equally spaced in frequency. The frequencies are chosen so that at least some of them correspond to communications channels or to molecular absorption signatures. The gain spectrum 71 of amplifying mirror 20 is substantially uniform across a considerable portion of the mode spectrum, which would normally give rise to multi-mode oscillation and/or unstable competition among the modes. The frequency selective device 30 has a transmission spectrum 72 that has a maximum for only one mode 73 within the gain band of the amplifying mirror 20. The combination of the gain of the amplifying mirror and the transmission of the frequency selective element causes the selected mode 73 to rise in power until it depletes all the inversion available in the homogeneously broadened gain medium. Thus only mode 73 oscillates, and does so stably, without competition from near-threshold modes. It is not necessary that the frequency selective element 30 act as a high contrast or narrowband filter. It is sufficient that the net unsaturated gain of one preferred mode exceed the net unsaturated gain of each of the other modes. An electronic signal applied to the mode selective element 30 via cable 40 causes the peak of the transmission of the mode selective element to rapidly switch from the oscillating mode 73 to any other mode within the gain spectrum 71. That switch extinguishes oscillation in mode 73, but laser radiation is not produced in any of the other modes until: sufficient pump power is absorbed to create an inversion sufficient to produce non-zero net gain in any of the previously-sub-threshold modes; the peak transmission of the mode selective element arrives at the frequency of another mode; and photons spontaneously emitted at the new mode frequency make sufficient cavity round trips under low-loss, high-gain conditions to build up significant intra-cavity laser power. Because the VECSEL cavity has a high Q (low loss) and the amplifying mirror has low gain, this third requirement takes longer than the second. As the mode selective element peak changes frequency, there is insufficient time for cavity power build up to occur in the modes between the initial mode and the final mode. Alternatively, the energy input (e.g. from a pump laser) to the amplifying mirror may be briefly reduced whenever the peak transmission of the mode selective element traverses the frequency of an unwanted mode. This ensures that the unwanted mode does not reach threshold. FIG. 13 illustrates an amplifying mirror 20, containing a gain region 21 and a Bragg reflector region 22, both fabricated from semiconductor materials such as InP, InGaP and InGaAsP. The amplifying gain region 21 may comprise layers of quantum-well material separated by layers of material transparent over the gain bandwidth 71 of FIG. 12, although not necessarily transparent at a pumping wavelength. The quantum well layers are at the anti-nodes of the cavity axial mode created by reflections from Bragg reflector region 22 and concave mirror 25 shown in FIG. 11. Although reflectivity of reflector region 22 is greater than 95 percent, it is necessary to suppress, by a factor of >10 4 , wavelength-dependent variations in net reflectivity due to reflections from surfaces behind the reflector region 22. In FIG 13, the back surface 23 of the mirror 20 is tilted to prevent such reflections propagating back into the optical cavity. Other means of suppressing interferences due to wavelength dependent reflectivity variations are possible, including depositing a completely opaque film on back surface 23. It is also desirable to suppress any phase variation of the reflection from that medium, i.e., 22 plus 23 such as depositing an opaque layer close to the outermost layer of the reflector region. The surface of the amplifying mirror inside the resonator 24 is also positioned or treated (e.g. coated) to suppress wavelength-dependent reflectivity over the gain band of the amplifying mirror. In FIG. 14 amplifying mirror 20 is excited by a pump laser beam 13 focused thereon by a lens 12 outside the cavity formed by mirrors 20 and 25. The pumped spot on mirror 20 is chosen to ensure single transverse mode operation. This can be achieved for example by using a pump beam which has: a transverse electromagnetic zero/zero mode(TEM 0 o); a circular projection spot onto amplifying mirror 20; and such projection spot being of such a size as to permit lasing only in a TEM 0 o transverse mode but not in any higher order transverse modes. Other optical pumping geometries are possible, including some with multiple beams and others that would access gain region 21 through reflector region 22 of FIG. 13. Beam 13 has a wavelength that is absorbed in the gain region thereby causing population inversion in the quantum structures, thereby producing gain. The same electrical pumping of an amplifying mirror may be used for a VCSEL without an external (extended) cavity. A mode selective element 30 of FIG. 15 includes a thin plane Fabry Perot tilted etalon 31 which transmits light at the mode desired for oscillation, but reflects sufficient light at other frequencies to discourage oscillation. This element can be used to provide a signal showing the degree of mutual detuning of tuning means and cavity modes for any given control voltage without perturbing the operation of the laser. This will allow the tuning means transmission maximum to be kept in coincidence with the cavity modes, and at the same time to compensate for any thermally induced change in the optical element. This property is achieved due to — and being equal, where n is the index of refraction. The spacer medium of dT dT the etalon is an electro-optical material bounded by conductive, partly transmitting mirror surfaces onto which a voltage can be applied by means of cable 40. The index of refraction n of the spacer medium varies with applied voltage, according to: n(V) =n 0 +λ 0 /2d N/N π or n(N) =n 0 -n 0 3 rN/2t, where n 0 is the index of refraction of the spacer medium with no applied voltage, N is the applied voltage, N π is the voltage necessary to achieve a π radian change in optical phase, t is the etalon thickness, λ 0 is the center optical wavelength of the gain band, and r is the electro-optic coefficient of the spacer medium. Thus the shift in peak wavelength of the transmission function for the tilted etalon is: Δλ(N, θ) = -λ 0 n 0 4 rN/2d(n 0 2 -sin 2 θ) "1/2 . Because electro-optic media are typically birefringent, it may be necessary for the light within such a frequency selective element to have a single polarization. This can be achieved by means of a polarization selective element 32. Adequate low-loss polarizers are known and include prisms with Brewster angled surfaces, thin films with polarization sensitive reflection coefficients and Glan-Thompson birefringent polarizing prisms. In order for the etalon 31 to have only a single transmission maximum within the gain band of the amplifying mirror (FIGJ3), the spacer thickness t must be less than λ 0 2 /(2n 0 Δλ G ) , where Δλo is the wavelength band producing gain (71 in FIG. 12). The spacer thickness t is small, and the voltages required by conventional electro-optic crystals to tune by changing refractive index over Δλo (~Nπ) may be prohibitively high. However, nematic and smectic liquid crystal media capable of adequate change in refractive index at low voltage are known and used for example in variable waveplates, choppers and displays. In such devices, an applied AC or DC voltage changes the orientation of the rod-shaped molecules of the liquid crystal medium in a plane defined by the incident light polarization and direction of propagation, thus varying the index of refraction. While the scattering loss of such media may be too high for some applications in high finesse, high-contrast Fabry Perot filters requiring high reflection (R ~ 95 percent) mirrors, the frequency selective element of this example has low finesse and low contrast and only requires relatively low reflectivity mirrors. Some loss due to scattering in the spacer medium is acceptable. The use of such liquid crystal media in step tunable external cavity semiconductor lasers is novel. While current nematic liquid crystal materials require milliseconds to change index of refraction, smectic-C materials are known that change orientation (i.e. index of refraction) in a few microseconds, thus adequate switching speed is obtainable. Alternatively, one can make use of the Vernier effect in which the modes of a thick etalon are spaced somewhat differently from a multiple of the spacing of the modes of the overall cavity, but lasing occurs only on the cavity mode most nearly centered on an etalon peak. The thickness of such an etalon may be greater than stated above, with a corresponding reduction in the necessary applied voltage. Etalon 31 may contain air (n 0 ~l), rather than a solid or liquid medium, between the conducting mirror surfaces. Previously a fixed spacing has been provided between the mirrors, or they have been actuated by piezoelectric transducers. Such etalons cannot switch frequencies rapidly enough (e.g. < 1 millisecond) for use in telecommunication applications. In the present invention, the mirrors are preferably free-standing semiconductor film stacks supported on flexure mounts and fabricated by deposition and etching in a planar MEMS (micro electro-mechanical systems) process. A voltage difference applied to the mirrors causes them to attract one another, changing the spacing between them, thereby varying the peak transmission frequency of the frequency selective element 31. Individual high reflectivity free-standing films with flexure support have been fabricated as laser mirrors for VCSELs. However, the use of such films in an intra-cavity etalon of a STECAML is novel. In FIG. 16 an electro-optically tunable Lyot filter determines the oscillating mode. The polarization selective component is a Brewster-angle window 33 which transmits essentially 100 percent of light linearly polarized in the plane of the page, but less of the light polarized in the perpendicular plane. The Lyot filter comprises a birefringent crystal 35 with ordinary 36 and extraordinary 34 axes oriented at ±45° to the high transmission direction of the polarization selecting element 33. Electrodes are deposited on the transverse faces of the crystal so that an electric field can be created within the crystal along the extra-ordinary axis 34 by applying a voltage V to the attached wires 40. Such an applied voltage modifies the indices of refraction of the crystal approximately according to: n 0 (V) = n 0 - n 0 3 r 13 V/2t and n e (V) = n e - n e 3 r 33 V/2t where t is the thickness of the crystal along the axis 34 (between the electrodes). More generally, the voltage may be applied along any crystal axis which exhibits electro-optic activity (i.e. an axis with non-zero electro-optic coefficient). The crystal 35 with length d along the light propagation axis acts as a high-order wave plate with retardation (in waves) of, N(V)= [n e (V)-n 0 (V)]d/λ. When N(V) < λ 0 /(2Δλo), there is only one transmission maximum for light of wavelengths within the gain bandwidth making a full round-trip through the filter. This defines the maximum crystal length d. The faces of crystal 35 are necessarily substantially perpendicular to the propagation direction of the light in the cavity and require anti-reflection coating to prevent their affecting the cavity resonances. Coatings with reflectivities <0.1 percent are known in the art. FIG. 17 illustrates the polarization of light in a plane perpendicular to the cavity axis in FIGS. 11 and 14 when using the Lyot filter of FIG. 16. In FIG. 17, the high transmission direction of the polarization selective element 33 of FIG. 16 is labeled "H" and shown as item 33a. The orthogonal low transmission axis is shown as "L" and 33b. Light of the high transmission polarization "H" having propagated from right to left through the element 33 in FIG. 16 encounters crystal 35. The light of the "H" polarization is resolved into components on "e" the extra-ordinary 34 and "o" on the ordinary axis 36 of crystal 35. In propagating through the crystal, these two polarization components accumulate a phase-shift with respect to one another. Light propagating out of the crystal to the left then encounters the amplifying mirror of the laser cavity (20 in FIGS. 11 and 14), which does not alter the accumulated phase shift, but reflects the light back through the crystal 35 from left to right. A unique property of the VECSEL amplifying mirror (unlike edge emitter mirrors) is that it does not significantly affect the polarization of the amplified beam. In an ECSAL the gain medium can also be polarization insensitive. The net effect is that the two polarizations of light with wavelength λ accumulate a phase shift of 2N(V) (in waves) with respect to one another as the result of a round trip through crystal 35. If 2N(V) is an integer, the light is returned to the polarization selective element in the polarization "H" with which it began. This condition represents the maximum transmission of the filter, and the axial mode with the wavelength λ that corresponds most closely to it will be the mode that lases. In general 2N(V) will not be an integer and the light will return to polarization selector 33 in an elliptical state of polarization shown graphically as 39 in FIG. 17. Such a polarization is not transmitted without loss through the polarizer 33 in FIG. 16. Additional loss occurs when that light is reflected by cavity mirror 25 in FIGS. 1 and 4 back through polarizer 33. When the voltage Vis changed rapidly to V, the selected wavelength λ also changes rapidly. The oscillating mode (73 in FIG 12) is extinguished and the sub-threshold mode with wavelength λ ' that makes 2N(V) most nearly integral begins to build up. FIG. 18 illustrates another embodiment of the mode-selective element 30 wherein the birefringent electro-optical media are constituted as two crystals (35 a and 35b). The extraordinary axis.34a of one crystal is parallel to the ordinary axis 36b of the other crystal and vice versa (34b and 36a). The net effect of having these crystals rotated by 90° with respect to one another is that the birefringences of the two crystals act in opposition, producing a net phase shift for round trip light of: 2N x (N a , V b )=2{[n e (V a ) - n 0 (V a )]d a /λ - [n e (V b ) - n 0 (V b )]d b /λ} . where d a and d b are the axial lengths of crystals 35a and 35b, respectively, and V a and V are the voltages applied between electrodes on opposing faces perpendicular to the extraordinary axes on those crystals, respectively. Said voltages are applied by means of wires 41a and 41b attached to the + electro-optic faces of the two crystals and wires 42a and 42b attached to the - electro-optic faces. Because of the cancellation in the phase shifts produced by the two crystals, considerably longer crystal lengths d a and d b or larger birefringences n e - n 0 can be employed without violating the "single transmission maximum" condition: Ν x (V a ,V b ) < λ 0 /(2Δλ G ). When V = V a = -V b the electro-optic phase variations of the two crystals add, even though the static birefringences subtract. Such a voltage arrangement can be obtained by connecting wires 41a and 42b to one side of the electro-optic driver circuit producing mode-selection voltage V while connecting 41b and 42a to the other side. Changing V to V 1 then has the same effect on the modes of oscillation as in FIG. 16. In particular the voltage-induced polarization changes illustrated in FIG. 17 again apply. The two crystals in FIG. 18 will require an anti-reflection coating to prevent their faces, which are substantially perpendicular to the light propagation direction, from altering the performance of the resonator. Reflections from the surfaces between crystal 35a and 35b in FIG 18 can be suppressed using known index matching material or optical contacting. In FIG. 19 a birefringent optical element (37) without electro-optic function is utilized. Birefringent element 37 is a zero-order half wave plate with extra-ordinary axis 38 oriented at 45° to the crystal axes. Such a device allows the ordinary axes 36 of the two crystals to be oriented parallel to one another at ±45° to the high- transmission axis of 33 and the extra-ordinary axes 34 to be oriented either parallel or anti-parallel to one another. The relative phase shifts due to the static birefringences of the two crystals subtract, as in FIG. 18, and proper connection of the leads attached to the electrodes on the electro-optic crystals allows the voltage-dependent phase shifts (and thus the tuning effects of the two crystals) to add. Adding non-electro- optic birefringent elements to the electrically controllable Lyot filter as illustrated in FIG. 19 can improve performance by, for example, broadening the angular acceptance of element 30 and reducing the required assembly precision. Functionally equivalent layouts to those illustrated are possible. FIG. 20 illustrates the effect of the half wave plate 37 in FIG 19 with extraordinary axis 38 and ordinary axis 39. The half wave plate has the effect of reflecting an arbitrary polarization 43 in one crystal through the plane defined by the extraordinary axis 38 and the propagation direction (out of the plane of the paper in FIG. 20) leading to an output polarization 44 which propagates into the second crystal. If the axes 38 and 39 of the waveplate 37 are oriented at approximately 45° to the axes 34 and 36 of the crystals 35a and 35b, then the projections of the arbitrary initial polarization 43 on the ordinary axis 36 and the extra-ordinary axis 34 of one crystal are exchanged by the wave-plate induced transformation of polarization 43 into 44. Thus the projection of polarization 43 on the ordinary axis 36 in the first crystal becomes the projection of polarization 44 on the extra-ordinary axis 34 in the second crystal. Thus even though the axes of the two crystals in FIG. 19 are parallel, the polarizations behave as they do in FIG 18 where the crystals are rotated 90° with respect to one another and the previous formula for N X (V) applies. In a further implementation, the bulk of the Lyot filter action of the mode selective element may be supplied by a uniaxial material without electro-optic properties while a variable waveplate with a range of 0-1 waves of retardation provides the tuning. In such a device one crystal in FIG 18 (e.g., 35a) would be made of calcite or a similar uniaxial substance, but would lack wires and electrodes (41a and 42a) and would have a length sufficient to give rise to a phase shift (in waves) between light polarized along the ordinary and extraordinary axes of N a =(ne - n 0 )d/λ min < λ min /(2Δλ G ), where λ m i n is the minimum wavelength of the desired tuning band. The second crystal modulator would be replaced by a thin variable waveplate having a variable retardance 0 < N b (V) < 1 and oriented to add its retardance to N a . Alternatively N a may be any N wherein λ G in the above equation is any λ within the desired tuning range. N must span a range of one unit which includes the value 0, but the range need not be symmetrical around 0. The total retardance of the combination would be N t (V) = N a + N b (V) < λ m j n /(2Δλ G ), thereby assuring a single value transmission peak as previously described. At N m j n , the retardance N t (V m i n ) is a half integer for the minimum wavelength λ m i n . Increasing the retardance of the electro-optic modulator by increasing the voltage then requires an increased value of λ to maintain a half intregal value of: N t (N) = (n e - n 0 )d/λ + N b (N), thus tuning the transmission maximum. The variable waveplate may comprise liquid crystal media with longitudinal AC and DC electric fields applied through transparent electrodes. Such a system can be compact and have a larger aperture than the previously described electro-optic wave plate. Smectic-C liquid crystals, for example, can be used to produce analog switching times below 1 millisecond. It is desirable to minimize the applied voltage integrated over time, in order to avoid damaging the electro-optic material or coating. Birefringence of electro-optic crystals can be tuned with temperature as well as with voltage. Thus, the element may be switched quickly (i.e. < 1 millisecond) to a desired wavelength by changing the voltage, and then the temperature adjusted to maintain that wavelength at zero voltage. The temperature will change slowly (» 1 millisecond), and the applied voltage should be changed gradually as the temperature changes so that the tuning wavelength is maintained. Switching quickly to a new desired wavelength at any time during the temperature change, or after, is still possible by a change in the voltage. After each switch of the wavelength, the temperature should be adjusted toward the new target. The voltage requirements of the crystals 35 in FIGs. 16, 18 and 19 in which the voltage is applied transversely across the crystals can be reduced by reducing the separation between the electrodes connected to the leads 40, 41 and 42, necessarily reducing the clear aperture of the device 30. In order to reduce loss due to scattering of light by the edges of the crystal, it may be desirable to create a secondary beam waist within or near the crystal 35. The first beam waist is inherently formed within the active mirror gain medium. In FIG. 21 an anti-reflective coated lens 61 has been added to mode selective element 30 within the cavity formed by amplifying mirror 20 and outer mirror 25. Lens 61, merely facilitates lower cavity loss. In FIG. 22 a secondary beam waist, which minimizes the cavity beam within the electro-optic crystal 35, has been created by a mirror 62. Because the light incident on the mirror is reflected at an angle, the dielectric coatings of this mirror can be designed to have less reflectivity for one polarization than the other. Thus mirror 62 can provide a secondary beam waist and a polarization selective element (33 in FIG. 21). Preferably the reflecting surface of mirror 62 has the FIG of an off-axis paraboloid or ellipsoid, thus minimizing wavefront distortions for the TEM00 transverse mode of the cavity formed by mirrors 20, 62 and 25. Mirror 25 may be concave, or planar in which case the second beam waist is at nearby mirror 25 rather than within crystal 35. Amplifying mirrors as illustrated in FIG. 13 can also have polarization dependence. The roles of the polarization selecting element 33 in FIG. 14 and the amplifying mirror 20 can be combined in a single element. Alternatively, the polarization dependent gain of the amplifying mirror 20 can act as a second polarization-selective element in a cavity containing a mode selector 30 with its own polarization selector (32 in FIG 15, for example). A cavity with multiple polarization selective elements requires care in aligning polarization axes to provide minimum loss and adequate tuning. Different geometries from those described may be used. In particular, the extra-ordinary axis of a uniaxial electro-optic crystal need not be oriented perpendicularly with respect to both the ordinary axis and propagation direction in order for the mode selective device to have the rapid electro-optic switching behavior described. It may be sufficient that the extra-ordinary axis lies at some angle in a plane orthogonal to the ordinary axis, with that plane also containing the axis of propagation. Additionally, electrical fields may be applied in different directions to switch the electro-optic media according to this invention. Frequency selective elements combining reflection (as in the etalon case of FIG. 15) and Lyot filter action (as in FIG. 16) are also useful. The electronically-controllable intracavity frequency-selective elements disclosed herein are intended as illustrations, and other types of filter may be used. In particular, a vertical coupler filter (VCF), which selects wavelengths through electronically tunable couplings between different spatial modes may be used instead of, for example, a Lyot filter, which acts on polarization modes. The VCF of U.S. Patent No. 5,668,900 may be used. In a preferred VCF the two modes correspond to separate waveguides with different effective indices of refraction and phase velocities (whereas a Lyot filter employs polarizations with different indices of refraction) and provides maximum transmission and feedback only for waves which transfer most efficiently from waveguide to the other. As in the case of a Lyot filter, the cavity mode or frequency corresponding to this maximum can be tuned by electronic means. Multiple electronically controlled intracavity frequency selective elements may be incorporated into laser cavities with appropriate coordination and control. In addition to voltage, other parameters may be altered for the purpose of control, including angle and temperature.

Claims: What is claimed is: 1. An external cavity laser comprising: a first mirror; a homogeneously broadened gain region comprising multiple quantum wells, having a gain region optical axis, and having a length equal to a selected integer multiple of a selected design laser wavelength, where the first mirror faces and is adjacent to the gain region; a second mirror, spaced apart from and facing the first mirror, with the gain region therebetween; and at least one of: an intra-cavity frequency-selective optical element; and a spacer configured to provide a selected mirror spacing between the first and second mirrors and to define an external cavity having a cavity length that provides at least first and second laser frequencies corresponding to a selected optical channel spacing. 2. The laser of claim 1, wherein said gain region is fabricated monolithically with said first mirror. 3. The laser of claim 1, wherein said first mirror has a reflectance of at least 90 percent. 4. The laser of any of claims 1-3, wherein said first mirror comprises a multilayer Bragg reflector. 5. The laser of any of claims 1-4, configured to receive optical energy from a laser pump, from a direction oblique to an output beam axis of said laser. 6. The laser of any of claims 1-5, further comprising a heat sink attached to said first mirror. 7. The laser of claim 6, further comprising a selected metal layer secured to said heat sink, wherein said first mirror has a reflectance of at least 99 percent and a combination of the metal layer and said first mirror have a reflectance of at least 99.5 percent. 8. The laser of any of claims 1-7, wherein said multiple quantum wells include at least one pair of quantum wells located at a peak amplitude location for a standing wave present in said gain region at said design wavelength. 9. The laser of any of claims 1-8, further comprising an anti-reflecting coating having a low reflectance at said design wavelength at a surface of said gain region facing said second mirror. 10. The laser of any of claims 1-9, wherein said second mirror is located by said spacer at a distance of between 0.5 mm and 10 mm from said gain region. 11. The laser of any of claims 1-10, further comprising an optical pump positioned to provide optical radiation for said gain region.. 12. The laser of claim 11, wherein said optical pump is a laser diode pump and said optical pump radiation is received at an anti-reflective coating on an external surface of said gain region at an equivalent Brewster's angle, relative to said gain region axis. 13. The laser of any of claims 11 and 12, wherein said optical pump is integral with said spacer or is aligned with and secured in a side wall of said spacer. 14. The laser of claim 6, wherein at least one of said optical pump and said laser provides an optical beam to which said heat sink is substantially transparent. 15. The laser of any of claims 1-14, further comprising a thermoelectric cooler/heater, in thermal communication with said heat sink and configured to control an operating temperature of said gain region to facilitate single mode-to-single mode tuning of said laser. 16. The laser of any of claims 1-15, further comprising a piezoelectric element mounted on said heat sink, wherein said spacer and the piezoelectric element are connected so that said length of said external cavity can be change to thereby tune a wavelength of an optical beam emitted by said laser. 17. The laser of any of claims 1-16, wherein at least one of said first mirror, said gain region and said gain region anti-reflection coating is formed by at least one of molecular bean epitaxy and metal oxide chemical vapor deposition. 18. The laser of any of claims 1-17, wherein said laser operates in at least one of a single transverse mode and a longitudinal mode that corresponds to a selected frequency used in optical communication. 19. The laser of claim 18, wherein said gain region is pumped to transparency and said external cavity is a stable resonator having a finesse at least equal to 10 and configured to suppress power in any side mode to a level no greater than 10 "4 of power in a selected central mode. 20. The ;laser of any of claims 1-19, wherein said gain region comprises at least one of AlJJij. y Ga^As^ and ImG .y As j.y P^ , where x and y are selected fractions satisfying 0 < x < 1 and 0 < y < 1. 21. The laser of any of claims 1-20, wherein said gain region has a net gain that exceeds losses in said cavity over a frequency band whose bandwidth is less than a selected mode-to-mode spacing. 22. The laser of claim 21, wherein said laser is tunable from a first selected mode to a second selected mode and remains stable in the second mode. 23. The laser of any of claims 1-22, further comprising pump radiation absorption means, formed in or adjacent to said spacer, at an equivalent Brewster's angle for absorbing any of said pump radiation reflected from an external surface that receives said pump radiation. 24. The laser of any of claims 1-23, further comprising at least one of a thermoelectric cooler/heater and an intra-cavity etalon. 25. The laser of claim 24, wherein at least one of said thermoelectric cooler/heater and said intra-cavity etalon is controllable using a digital controller. 26. The laser of any of claims 1-25, further comprising a first reflective surface facing said gain region along said first optical axis, and a curved second reflective surface facing the first reflective surface along a second optical axis. 27. The laser of claim 26, wherein said first reflective surface comprises a mode-selective intra-cavity etalon. 28. The laser of any of claims 1-27, wherein said intra-cavity frequency- selective element comprises an electronically actuated, frequency-selective optical element within said cavity that induces optical loss for all but a selected potential mode of oscillation, that suppresses laser action on each loss-induced mode, and that switches from a first potential mode of oscillation to a second potential mode of oscillation. 29. The laser of any of claims 1-27, wherein said intra-cavity frequency- selective element comprises an electronically actuated, frequency-selective optical element within said cavity that reduces round trip optical gain for all but a selected potential mode of oscillation, that suppresses laser action on each mode for which round trip gain is reduced, and that switches from a first potential mode of oscillation to a second potential mode of oscillation in a time interval of length less than one msec. 30. The laser of any of claims l-29,wherein said first mirror and said gain region forma an epitaxially-grown, monolithic semiconductor structure, said gain region further comprises a plurality of active layers, spaced apart by spacer layers, arranged to provide optical gain in said cavity, and positioned adjacent to said first mirror, wherein at least two of the active layers have different indices of refraction, selected to provide optical radiation reflectivity at least equal to 95 percent within a selected optical bandwidth. 31. The laser of any of claims 1-30, further comprising a source of radiation that is directed onto at least one of said first mirror and said gain region. 32. The laser of any of claims 1-31, wherein said frequency-selective element comprises at least one of (i) a planar etalon including an etalon spacer comprising an electro-optically active material and (ii) a monolithic, planar, air-spaced etalon having at least one free-standing dielectric film that serves as a reflecting surface, wherein a length of the etalon spacer or a length of said cavity is chosen so that only one transmission maximum frequency lies within a selected gain band of the etalon. 33. The laser of claim 32, wherein said transmission maximum frequency is tunable over a selected frequency band by at least one of (i) application of a variable voltage to said spacer material and (ii) changing an angle of orientation of said etalon with respect to said gain region optical axis. 34. The laser of any of claims 33, wherein said etalon spacer comprises at least one of a nematic crystal and a smectic crystal. 35. The laser of any of claims 28 and 29, wherein said frequency-selective element comprises: a polarization-selective element that receives and passes a light beam; and a birefmgent electro-optical medium having ordinary and extraordinary axes oriented at 45° with respect to a selected transmission axis of the polarization-selective element, the optical medium having anti-reflection-coated surfaces oriented substantially perpendicular to a path of light beam propagation through the medium and having a light propagation length such that light having a selected initial polarization that passes through the medium along the selected transmission axis will be in a high transmission polarization state for at most one selected mode; and voltage means for applying a selected voltage to the electro-optical medium over a range of voltages. 36. The laser of claim 35, further comprising temperature control means, connected to said electro-optical medium, for controlling temperature of said medium. 37. The laser of any of claims 1-35, further comprising a beam focusing module for providing a secondary beam waist for a selected fundamental mode of said cavity. 38. The laser of claim 37, wherein said beam focusing module comprises at least one of (i) a lens or concave mirror with at least one multi-layer dielectric coating and (ii) an off-axis parabolic lens. 39. The laser of 35, wherein said electro-optical medium comprises at least first and second optical elements, with at least one birefringent element positioned between the first and second elements and oriented to increase angular acceptance of said frequency-selective element. 40. The laser of claim 39, wherein said birefringent element is a zero order, half wave plate, positioned between said first and second optical elements and oriented to rotate first and second directions of optical beam polarization with respect to a selected axis. 41. The laser of claim 35, wherein said electro-optical medium comprises at least one of lithium tantalate and lithium niobate. 42. The laser of claim 35, wherein said extraordinary axis is perpendicular to at least one of said anti-reflection coated surfaces and lies in a plane oriented at approximately 45° with respect to a plane defined by said high transmission polarization state axis of said polarization-selective element and a direction of light propagation through said frequency-selective element. 43. The laser of claim 35, further comprising: an electro-optically active medium that provides a variable retardation of speed of propagation of light having a first polarization relative to speed of propagation of light having a second polarization, the retardation being controllable by an applied electrical voltage, whereby a combination of said birefringent medium and the electro-optically active medium allows selection of a wavelength of light that returns to said polarization-selective component in said high transmission axis by application of a voltage to the electro-optically active medium. 44. The laser of claim 43, wherein said electro-optically active medium comprises at least one of a nematic crystal and a smectic crystal. 45. The laser of claim 35, wherein said electro-optical medium comprises an electro-optical crystal. 46. The laser of any of claims 1-45, wherein light reflected into a fundamental lasing mode from said first mirror and produced by any medium other than said gain region is reduced by a factor of at least 10 4 in power relative to a power level of light produced in said gain region. 47. The laser of any of claims 1-46, wherein said second mirror is planar and additionally comprises a focusing element, positioned within said cavity and configured so that said cavity forms a stable resonator centered within a stability range of said cavity. 48. The laser of any of claims 1-47, wherein said second mirror is concave. 49. The laser of claim 48, wherein said second mirror has a radius of curvature selected to maintain a single transverse mode of oscillation over a range of optical pumping power and is coated with at least two layers of dielectric materials having different indices of refraction to provide reflectance at least equal to 95 percent over a selected gain band of said second mirror. 50. The laser of any of claims 1-49, wherein at least one of said first mirror, said gain region and said spacer is attached to a heat sink. 51. The laser of any of claims 1-50, further comprising a housing that localizes at least one of said first mirror, said gain region, said second mirror and said frequency-selective element. 52. The laser of claim 35, wherein said electro-optical medium comprises at least one surface having an anti-reflection coating. 53. The laser of any of claims 1-52, further comprising a diode pump laser, aligned at an equivalent Brewster's angle relative to said optical axis and being integral with said spacer, wherein said spacer has a light absorption element. 54. The laser of any of claims 1-53, further comprising: wavelength tuning means for tuning said laser from a first mode to a second mode; an optical modulator that receives an optical beam from said laser and impresses an information-carrying modulated signal upon the received beam; and an optical coupler for receiving and coupling the modulated signal into an optical fiber. 55. A method for making a semiconductor structure for use as part of a laser, the method comprising: forming a semiconductor structure, using at least one of molecular beam epitaxy and metal oxide chemical vapor deposition, the structure comprising: at least two semiconductor crystalline layers on a semiconductor substrate; a multi-layer mirror positioned facing the crystalline layers; a multi-layer active gain region having at least two quantum wells and having a thickness an odd integer times a selected design wavelength for the laser; and an anti-reflection coating having a low reflectance at the design wavelength, removing the substrate to provide a modified semiconductor structure; and providing a heat sink for at least one of the mirror and the gain region. 56. A method of calibrating mode spacing of a laser, the method comprising the steps of: providing a laser external cavity and positioning a first mirror at an end of the cavity, where the cavity has a selected length that defines a plurality of optical modes, with each mode having a corresponding wavelength; providing a homogeneously broadened gain region, located adjacent to or within the external cavity, that is active over a wavelength band having a bandwidth less than a mode-to-mode spacing, where the gain region is tunable from a first mode to a second mode and is stable at the second mode; providing optical tuning means, including wavelength-selective optical sensing means for tuning the laser from the first mode to the second mode and for detecting at least one pulse indicating a transition from the first mode to the second mode; providing a mode controller for maintaining the laser in a selected mode; varying a control parameter associated with the controller to sweep the tuning means between a selected longest wavelength mode and a selected shortest wavelength mode that can be generated by the laser; recording in a memory associated with the controller a transition control parameter associated with the optical sensing means, when the optical sensing means detects a mode-to-mode transition; and determining and storing at least one single mode value as an approximate half- increment magnitude between magnitudes of transition control parameter values for at least two adjacent modes.

Inventors: GARNACHE ARNAUD, KATCHANOV ALEXANDRE, KNIPPELS GUIDO, LEVENSON MARC, PALDUS BARBARA, RELLA CHRIS, RICHMAN BRUCE, ROMANINI DANIELE, STOECKEL FREDERIC, GARNACHE, ARNAUD, KATCHANOV, ALEXANDRE, KNIPPELS, GUIDO, LEVENSON, MARC, PALDUS, BARBARA, RELLA, CHRIS, RICHMAN, BRUCE, ROMANINI, DANIELE, STOECKEL, FREDERIC
Applicants: BLUE LEAF INC, BLUE LEAF, INC.
Application year: 2001
Main classification: H01S
IPC codes: H01S5