Producing laser light of different wavelengths

In a tunable laser device a plurality of basically identical lasers are arranged adjacent to each other in a line or row on a common substrate. The lasers can be DFB-type and they have different emission wavelengths, obtained from e.g. different pitches of gratings which define the wavelengths of the respective lasers. The lasers can be activated to emit light independently of each other by supplying electrical current to contacts located on the top sides of the lasers. When a laser is activated, the other lasers are biased, so that lasers located at one of side of the active laser will be transparent to the emitted light, which can then travel from the lasers through an electrooptic modulator, and the lasers located at the other side will absorb the light. By controlling the temperature the wavelength of emitted light can be finely adjusted. Such a laser device has a compact structure, is fairly insensitive to variations of the used drive currents and can in simple way be adjusted to different operating conditions and be compensated for aging effects.

The invention relates to a light source for producing or emitting light,
 the wavelength of which can be controlled such as by means of electrical
 signals, in particular a laser device which is tunable to emit light of
 different wavelengths, and also to a method of producing or emitting laser
 light of different wavelengths.
 BACKGROUND
 By introducing the use of wavelength division multiplexing (WDM) in optical
 fiber communication networks the bandwidth of and thereby the transmitted
 information amount in such networks can be made much higher than before,
 without using extremely high transmission rates. The information is
 instead transmitted on a number of parallel channels which each one
 comprises a definite, separate wavelength region or wavelength band.
 Systems are is presently introduced comprising 4-16 channels having a
 transmission rate or bit rate of 2.5 Gbits/s per channel. As regarded in a
 longer perspective of time, certainly still more channels will be used.
 Thus, it is completely realistic to use a number of 16-32 channels and in
 laboratory situations functioning transmission systems using 128 channels
 have been demonstrated. Further, in the same way, certainly also the bit
 rate per channel will be significantly increased, for example up to 10
 Gbits/s. Still higher transmission rates have been used in laboratory
 situations such as bit rates of 20, 30 and 40 Gbits/s and they will
 perhaps also be used in the future.
 For each channel and wavelength region in wavelength multiplexed
 transmission a separate light source such as a suitable semi-conductor
 laser must be used, the light issued by the laser in addition having to be
 capable of being modulated in order to obtain a bit stream carrying useful
 information. However, one of the main problems is to achieve such laser
 transmitters, since they must have a narrow optical line width, i.e. have
 a small chirp. It can be accomplished by among other methods introducing
 external modulation, i.e. that the laser is driven by a constant current,
 and by making the modulation by means of a separate intensity modulator or
 an intensity modulator monolithically integrated with the laser, for
 example of electroabsorption type. The laser should be either type DFB,
 i.e. a laser having distributed feedback, or of type DBR, i.e. a laser
 having a distributed Bragg reflector, in order to ensure that when
 operating the laser only one longitudinal electromagnetic mode lases.
 The wavelength region which presently is most interesting for wavelength
 multiplexing comprises the range of about 1530-1560 nm. This is the range
 for which good fiber amplifiers are available, such as erbium doped fiber
 amplifiers (EDFA:s). In the future other wavelength ranges can start to be
 used such as for example about 1300 nm.
 Typically, presently used light emitting and modulating devices are
 constructed so that laser transmitters of e.g. type DFB are manufactured
 for different wavelengths, at which the respective laser transmitter can
 be activated for emitting light. The lasing wavelength of such a DFB laser
 is determined by the active refractive index in the active layer of the
 laser and of the pitch ("pitch") of the longitudinal grating, i.e. of the
 grating period. Such a laser can be tuned by controlling the temperature
 of the laser within a wavelength interval of about 5 nm, since in the
 typical case the wavelength varies by about 0.1 nm/K and since
 semiconductor lasers cannot be operated at too high temperatures due to
 increasing threshold current and a reduced output power of the emitted
 light for increasing temperature. It means that lasers have to be
 manufactured in different wavelength classes and that when installing
 transmitter equipment for wavelength multiplexing correct components have
 to be selected. It also means that the emitted wavelength cannot be easily
 changed within a larger wavelength range, i.e. a change to an arbitrary
 channel cannot easily be made. Possibly only a change of channels can be
 made for lasers operated at wavelengths which are close to each other.
 However, such channel changes can be of interest in flexible optical
 networks comprising optical cross connections (OXC) and optical
 multiplexers having an add and drop function (OADM, Optical Add/Drop
 Multi-plexers).
 Different proposals have been presented in order to achieve lasers having a
 wider range in which the wavelength can be selected. These proposals
 comprise different variants of DBR lasers, in which the reflection maximum
 of the grating can be displaced by injecting current or by heating the
 wave guide locally or by subjecting the device to an electrostatic field.
 One proposal is based on the method that a DFB laser is divided in
 different segments and the current is varied in the different segments. A
 third proposal is based on the method that the laser cavity is divided in
 different subcavities having somewhat different lengths and interference
 is used between the different cavities in order to define the wavelength
 which is to be emitted, so called Y-lasers or C.sup.3 -lasers. A problem
 associated with all these types is that the tuning mechanism is relatively
 complicated such as that it requires complicated control algorithms and
 that all those types which are based on current injection in order to
 change the refractive index, potentially suffer from problems associated
 with the reliability of the devices.
 SUMMARY
 It is an object of the invention to provide a laser device which can be
 tuned to provide light of different wavelengths within a not too limited
 wavelength range.
 It is another object of the invention to provide a tunable laser device
 which has a reliable function and is not too sensitive to the choice of
 the operating voltages and operating currents.
 It is another object of the invention to provide a tunable laser device
 which has a compact construction and can be built in a monolithically
 integrated way on a single circuit plate and thus does not require
 additional optical components in order to operate.
 The problem which the invention intends to solve is thus to provide a
 tunable laser device which has a simple and reliable construction and
 function and which can easily be controlled to emit light at a selected
 wavelength within a not too limited wavelength range.
 The solution of the problem presented above and also other problems is to
 provide a number of independent lasers which in principle are identical to
 each other and are located adjacent to each other in a line or row
 configuration. The lasers have different emission wavelengths and can be
 operated to emit light independently of each other. The light emitting
 directions of all lasers substantially agree with each other, i.e. the
 lasers have the same longitudinal direction. Further, the arrangement of
 lasers is such that light emitted from a laser in the row will pass in a
 direction towards and/or through the other lasers and in particular the
 laser cavities thereof.
 Such a laser device is advantageously constructed by means of semiconductor
 lasers on the same semiconducting or other type substrate. Compared to a
 tunable laser arrangement comprising several lasers which emit light in
 parallel to each other and at the side of each other, in the laser device
 as described herein an optical coupler is not required which is a
 significant simplification of prior art devices.
 Generally, thus, the laser device as described herein is robust and simple
 in its construction. It can also easily be controlled since it requires
 only a relatively simple control algorithm. In the design of a laser
 device based on semiconductors the emission wavelengths of the laser can
 be finely adjusted by controlling the temperature of the device in the
 known way. Further, the device requires a small surface on the substrate
 common to the lasers since no coupler is required. The manufacture of the
 laser device can be made by means of the same known process which is used
 for manufacturing DFB lasers.
 Generally, for emitting laser light of one of a plurality of different
 wavelengths, thus the following steps are executed: first at least two
 laser units are provided which are adapted to emit light of different
 emission wavelengths; the laser units are then placed e.g. in a line or
 row, so that when one thereof is biased for emitting laser light, the
 light is emitted in directions, generally in two opposite directions, one
 of which will pass through at least one other laser unit--preferably all
 laser units are located, so that light emitted from all of the laser units
 have the same directions; only one, a first one of the laser units is then
 biased or activated to emit light, such as by providing it with suitable
 driving electrical voltages and currents; a second, different one of the
 laser units, through which one of the directions of the light emitted from
 the first laser unit passes, is biased either to be transparent to the
 light emitted from the first laser unit, i.e. to let the emitted light
 through, or to absorb the light emitted from the first laser unit.
 The laser units can be divided in second laser units located on one side of
 the first laser unit and third laser units located on an opposite side of
 the first laser unit, where possibly such second and third laser units do
 not exist, depending on the location of the first laser unit in the line
 or row of the laser units, so that one direction of light emitted from the
 first laser unit extends through all second laser units and an opposite
 direction of light emitted from the first laser unit extends through all
 third lasers. All the second lasers can then be biased to be transparent
 to the light emitted from the first laser unit and all the third lasers
 can be biased to absorb the light emitted from the first laser unit. The
 light emitted in one of the opposite directions from the first laser unit
 can thus always be absorbed by some suitable means, such as by a specially
 adapted absorbing unit.
 The temperature of the laser units, in particular in the case where they
 are based on semiconductors, can be controlled to a desired value in order
 to produce a fine adjustment of the emission wavelengths of the laser
 units. The light emitted by activating the first laser unit can be
 modulated in order to carry information bits.

DETAILED DESCRIPTION
 In FIG. 1 a cross section of a tunable laser device is shown which is
 constructed on a semiconductor plate being an n-doped InP-substrate 1. The
 laser device comprises a plurality of DFB lasers 3, 3', in the example
 illustrated three lasers, but in practice at least up to ten individual
 lasers can be used. The lasers are located in a row and adjacent to each
 other and are numbered 1, 2, 3 and all have different grating periods of
 their gratings 5. Each laser 3, 3' is operated basically independently of
 the other lasers, so that it can be made to emit laser light independently
 of the other lasers, if a suitable electric current is supplied thereto.
 The grating periods of the lasers are chosen in a suitable way so that the
 wavelengths at which the lasers emit-laser light will have a suitable or
 sufficient difference between each other.
 The wavelength separation between the individual lasers 3, 3' and the
 grating constants thereof, i.e. the coupling strength of the gratings per
 unit of length, must be selected in such a way that the stop band, which
 is the spectral region within which the grating reflects light of the
 laser, does not overlap the stop band of the other lasers. If this
 condition is not fulfilled, problems can be obtained related to
 non-desired parasitic reflections. The spectral width .DELTA..lambda. of
 the stop band, which also is the optical 3 dB bandwidth of the grating, is
 given approximately by
EQU .DELTA..lambda.=.kappa..lambda..sup.2 /.pi.n
 where .kappa. is the coupling strength in the grating, .lambda. is the
 wavelength of light in vacuum, and n is the effective refractive index in
 the waveguide. Typical values for DFB lasers are .kappa.=50 cm.sup.-1,
 which can vary from 10 cm.sup.-1 to 100 cm.sup.-1, .lambda.=1.55 .mu.m and
 n=3.25, which gives .DELTA..lambda..apprxeq.1.2 nm. In order to ensure
 that small reflections due to feedback from gratings located around a
 considered DFB laser 3, 3' will not interfere with that laser, therefore,
 for the values given as an example, the distance between the emission
 wavelengths of the lasers should be at least 2-3 nm, which corresponds to
 approximately 1.5.multidot..DELTA..lambda.. In the typical case, for
 maintaining a suitable overlap between those wavelengths which can be
 obtained by temperature control, see below, the difference between the
 wavelengths of light emitted from the lasers can however be somewhat
 increased and can be typically about 3-5 nm.
 All the lasers have a common ground contact 7 such as on the underside of
 the substrate 1. The waveguides 9 in the lasers are manufactured of
 InGaAsP (bulk material or quantum wells) having a luminescence wavelength
 of 1550 nm (Q1.55). Above the layers containing the waveguides are the
 longitudinal gratings 5 arranged. The grating period is determined when
 manufacturing the semiconductor plate by means of for example electron
 beam lithography. Each laser 3, 3' has its own electric contact 11 on its
 top side. The lasers 3, 3' are electrically separated from each other by
 means of trenches 13 containing for example semi-isolating InP, SI-InP.
 The lasers 3, 3' are optically connected to each other by means of passive
 waveguides 15 which are located at the bottoms of the trenches 13 and can
 be InGaAsP having a luminescence wavelength of about 1450 nm (Q1.45).
 Adjacent to a laser 3' at the end of the row of lasers 3, 3' can an optical
 intensity modulator 17 of electro-absorption type be arranged which
 comprises a p-doped InP-layer 19. This layer 19 is located on top of a
 passive waveguide 21 of the same type as those waveguides 15 which connect
 the lasers 3, 3' to each other. The InP-layer 19 of the intensity
 modulator has on its top side an electric contact 23 for supplying the
 modulating electric voltage.
 The operation of the laser device will now be described in conjunction with
 the diagram in FIG. 2. The current intensities which are supplied to the
 lasers 3, 3' are denoted by I.sub.1, I.sub.2,I.sub.3 where the index
 corresponds to the order number of the lasers. One of the lasers is
 selected, e.g. laser No. 2, by biasing it forwardly by a large current
 I.sub.2 =I.sub.las, which substantially exceeds the threshold current
 I.sub.th and which thus passes through the laser. The laser will then
 start emitting laser light. This light is emitted in two opposite
 directions, in the longitudinal direction of the laser, i.e. both in the
 forward direction, i.e. to the left, towards the modulator 17, and in the
 backward direction, i.e. to the left, as seen in FIG. 1.
 For the simplest type of laser design the ratio of light which is emitted
 in one direction and light emitted in the opposite direction is equal to
 one. However, the grating of the laser can be designed so that more light
 is emitted in one direction than in the opposite one. It is achieved in
 the known way by varying the strength of the grating in the longitudinal
 direction (in the same laser).
 Those lasers which are located behind the selected laser, in the mentioned
 example laser No. 1, are operated to be voltage biased in the backward
 direction or with only a weak biasing in the forward direction so that no
 electrical current passes through these lasers. The current I.sub.1 can
 thus in the example be negative or have a small positive value. These
 lasers located behind will then absorb the light which is emitted in the
 backward direction of the selected laser. Thereby problems are eliminated
 which are associated with reflections from the rear laser facet in the
 selected laser, i.e. from the rear side surface of the selected laser as
 seen in the longitudinal direction. In order to ensure that no reflections
 influence the rearmost laser--the rearmost laser is laser No. 1 in FIG.
 1--if this is selected for emitting light, an extra section, not shown,
 can be applied to the rearmost portion of the laser device. This section
 can have a construction identical to that of the lasers of the row but
 need not be capable of emitting light. This section is given a driving
 current so that it will be absorbing in the same way as has been described
 above for those lasers which are located behind an activated laser.
 Alternatively, at the rearmost portion of the laser device one or several
 dielectric anti-reflection layers, not shown, can be arranged.
 The lasers which are located in front of the selected lasers, is i.e. those
 which are located between the selected laser and a possible modulator, in
 the example chosen laser No. 3, are operated moderately biased in the
 forward direction. Thereby is meant that the voltage over these lasers in
 the typical case is selected so that the current has a value between the
 transparency current I.sub.tranp and the threshold current I.sub.th, see
 FIG. 2. In the example I.sub.tranp &lt;I.sub.3 &lt;I.sub.th thus should be true.
 The transparency current I.sup.tranp is defined as that current intensity
 at which an incoming light signal experiences neither a net amplification
 or a net absorption in the active layer of the laser. The threshold
 current I.sub.th is defined as that current intensity at which the
 stimulated amplification balances the total losses produced by both
 absorption and coupling of light away from the laser cavity and is thus
 the current at which a laser starts to emit laser light, i.e. starts to
 "lase", when increasing the current through the laser.
 The choice of the exact value of the current intensity for the lasers
 located in front of the selected laser is not particularly critical, since
 the interval between the transparency current I.sub.tranp and threshold
 current I.sub.th typically comprises several mA. The exact choice of
 operating current determines the level of the output power. Typically, the
 output power can be varied by a few dB depending on the position of the
 current intensity of these lasers within the interval [I.sub.tranpl,
 I.sub.th ].
 All of said three current intensities I.sub.las, I.sub.th and I.sub.tranp
 are rather strongly dependent of temperature. Generally therefore, when
 using semiconductor laser devices correct values must be selected. In an
 automatically operating device the lasers can then be controlled by means
 of some controlling means such as a microprocessor 100, in the memory 110
 of which tables are stored for the temperature dependency of these
 quantities. The control means must then also comprise some temperature
 sensor, such as temperature sensors 1, 2, 3 and 4 which may be of a
 construction known in the art, and produce a signal readable by
 microprocessor 100 and then selects, as guided by the measured temperature
 and table values and as commanded by suitable control signals, the correct
 operating currents to the lasers included in the laser device in order to
 activate the desired laser so that its light is emitted in the intended
 way. To provide such a control means results in no substantial
 complication compared to presently used systems having a similar function.
 Each construction of a wavelength tunable light source based on
 semiconductors thus requires some form of logical control mechanism.
 If the driving current of a laser located in front, i.e. laser No. 3 in the
 example, has a too small value, light will be absorbed therein and the
 total output power of the laser device decreases. It can be automatically
 detected by means of some suitable photo detector, not shown, which can be
 arranged after the modulator and is coupled to the control means.
 Alternatively, the modulator 17 can itself be used as a photo detector,
 since the light absorbed therein causes a photo current. By measuring this
 photo current one also measures the amount of light which passes through
 the modulator. For compensating such decrease the operating current of
 those lasers which are to be transparent can be increased or the current
 through the active laser can be increased. However, there is no problem
 associated with the laser making non-desired node jumps or getting
 instable, which is the case if unsuitable currents are selected in a laser
 which bases its tuning mechanism on changes of the refractive index as
 induced by the supplied electric current, such as DBR lasers and similar
 ones. When a laser ages, also the optimal operating currents will be
 changed, usually increase. This can with the construction described above
 easily be compensated in the indicated way, whereas in lasers of types
 similar to the DBR laser a more complicated monitoring is required in
 order to ensure that the laser device will not come in an operating state
 having unsuitable or incorrect current values, which for example can
 result in a bad side mode suppression of the laser.
 A fine adjustment of the wavelength of laser light emitted from the device
 described above can be achieved by changing the temperature of the whole
 circuit plate, e.g. by arranging a Peltier element, not shown, in a
 capsule accommodating the laser device. If one wants to be able to tune
 the laser device to an arbitrary wavelength within a certain wavelength
 interval, thus, the number of lasers and the differences between the
 grating periods thereof should be selected so that the possible
 temperature changes, for example within an interval from about 0.degree.
 C. to about 50.degree. C., is sufficient therefor.
 The laser device can in principle be manufactured in the same way as used
 when manufacturing DFB lasers, see e.g. the article "Zero-bias and
 low-chirp, monolithically integrated 10 Gb/s DFB laser and
 electroabsorption modulator on semi-insulating InP substrate", O. Sahlen,
 L. Lundqvist, S. Funke, Electron. Lett., Vol. 32, No. 2, pp. 120-121,
 1996, which is incorporated by reference herein. An InP-substrate (a
 semi-isolating InP-substrate or an n-doped InP-substrate can be chosen)
 can be used according to the above and different alloys of InGaAsP and
 possibly InAlGaAs for building the structure according to FIG. 1. The
 different layers can be grown epitaxially by means of MOVPE, Metal Organic
 Vapor Phase Epitaxy, or some variant thereof, or alternatively some
 variant of MBE, Molecular Beam Epitaxy. The device can be produced for use
 in the usual wavelength band of about 1550-1560 nm or by changing the
 material composition or the alloy contents in the InGaAsP-layers, to other
 wavelength ranges, for example to the wavelength interval around 1300 nm.
 Still smaller wavelengths, for example in the wavelength interval about
 980 nm, can be achieved by using other material combinations such as the
 InGaAs/GaAs/AlGaAs-system. Naturally, it is conceivable to also use other
 material systems than semi-conductors, such as for example doped
 dielectric materials, e.g. erbium doped quartz-on-silicon, or doped
 ferroelectric materials, such as erbium doped lithium niobate.
 A laser device according to the above has been manufactured comprising two
 cascaded DFB lasers which had a length of 400 .mu.m and had a shift of a
 quarter of a wavelength and a Franz-Keldysh (FK) modulator. The device was
 manufactured as described in the article mentioned above comprising
 electron beam lithography for defining the gratings, however with the
 exception that the active layers now comprise six quantum wells, "strained
 quantum wells". By adjusting the temperature each one of 11 wavelength
 channels can be selected, which had a frequency difference of 100 GHz,
 i.e. a tuning interval of more than 8 nm was obtained, compare the curves
 in the diagram in FIG. 3. The side mode suppression ration SMSR was better
 than 40 dB and the modular extinction ratio was larger than 11 dB for all
 temperatures, when the modulator was supplied with a voltage of 0 to -2 V.
 The temperature was varied within an interval of 277-324 K. The maximum
 current used did not exceed 100 mA, which produced a power output of the
 circuit plate exceeding 1 mW for all operating states. A typical power of
 the emitted light was 3 mW. The electrooptical small signal response of
 the modulator was 16 GHz. The laser device was tested in a system having a
 bit rate of 2.488 Gbits/s (corresponding to STM-16) for a length of 543 km
 of optical, not dispersion shifted standard fibers. In the diagram of FIG.
 4 curves are drawn for the bit error rate BER for four different channels.
 Two of the curves correspond to the case where the laser located closest
 to the modulator is activated whereas the two other ones correspond to the
 case where the rear laser is activated and the front laser is weakly
 forwardly biased. The modulation had a peak-to-peak value of 2 V in all
 these cases.