Abstract:
A low-loss comb-generating optical cavity including an optical amplifier and a microwave-driven electro-optic modulator crystal, produces a comb of optical frequency sidebands having spectral lines equally spaced around the frequency of an input laser beam incident on the comb-generating cavity. The comb-generating cavity includes an input mirror movable along the beam propagation direction, and a fixed position output mirror located at time synchronous distances of both the input laser wavelength and modulation wavelength. The comb-generating cavity and its microwave driven modulator are in resonance with the input laser beam, and provide iterative or recirculating beam action that transfers the input optical frequency of the laser, sideband by sideband, to remote and precisely known comb frequencies offset from, and centered on, the input laser frequency. Optical parametric amplification within the comb-generating cavity extends the sideband or comb spectrum and sharpens the time domain impulse represented by the cavity circulating fields. A relatively short bandpass filter optical cavity receives the comb output of the comb-generating cavity and is made up of the fixed-position mirror and a third mirror movable along the beam propagation direction. Fine movement of the third mirror tunes the bandpass filter cavity, and preferentially couples out the power of one or more comb frequencies. An optional input optical cavity at the input side may increase efficiency. A self-oscillating configuration provides optical parametric oscillation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Provisional patent application Ser. No. 60/072,243, filed Jan. 23, 1998, entitled OPTICAL FREQUENCY SHIFTER WITH OPTICAL GAIN by John. L. Hall, Jun Ye, and Long-Sheng Ma. 
    
    
     The United States of America as represented by the Secretary of Commerce, National Institute of Standards and Technology, may have rights under this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of optical frequency generators, and more specifically, to the generation of optical frequency combs. 
     2. Description of the Related Art 
     An Electro-Optic Modulator (EOM), when driven by an appropriate single Radio Frequency (RF) electromagnetic field, produces optical frequency light sidebands on an original single frequency light beam that traverses the EOM. The sidebands are equally spaced about the input beam. The spectral extent of the sidebands can be increased by recirculating the modulated light beam through the EOM, to thereby iteratively produce additional light sidebands on each daughter light beam that was generated by a previous interaction. In this way, an optical comb is built up, the spectral extension of which is limited by optical transmission losses, phase mismatching error associated with synchronization or length errors, and wavelength “breadth” induced phase dispersion of the EOM and its mirrors. (See, for example, “A Highly Accurate Frequency Counting System for 1.5 Micro Meter Wavelength Semiconductor Lasers’, PROCEEDINGS OF THE SPIE, Vol. 1837, 16-18 Nov. 1992, pp. 205-215, by M. Kurogi, K. Nakagawa, and M. Ohtsu, and “Optical Frequency Comb Generator”,  IEEE J. Quant. Electr ., Vol. 29, Oct. 1993 pp. 2693-2701 (1993), by M. Kurogi, K. Nakagawa, and M. Ohtsu. 
     FIG. 1 shows the output of such a prior comb generating cavity  60  that operates to generate an optical frequency comb  61  having sideband portions  62  and  63  that are centered upon the frequency  64  of an input laser  65 . Increasing frequencies within OFC  61  are shown by increasing values along the X axis, and the relative power in each comb frequency is shown on the logarithmic Y axis, the power of frequency  64  being the largest amplitude 
     In accordance with an aspect of the present invention, comb-generating cavity  60  includes an optical amplifier or optical parametric amplifier, and the utility of optical comb  61  is enhanced by the use of a resonant and tunable bandpass filter optical cavity that operates as a direct output coupler for comb-generating cavity  60 . This output coupler operates to increase the strength of a selected comb frequency component by several orders of magnitude. 
     A publication by John. L. Hall (“Frequency stabilized lasers—a parochial review”,  Proceedings Reprint, SPIE , Vol. 1837, 16-18 November 1992, pgs. 2-15, at section 5.4.2 on page 12) recognizes Kurogi, Nakagawa and Ohtsu as providing a microwave modulator that is enclosed in a low-loss cavity, wherein a sideband that is produced on the first transit is used as the source for a second sideband, and the second for a third, etc., whereby a spectral width of about + and −4 THz is provided, made up of individual lines spaced by the 5.6 GHz frequency. Hall then suggests “recycling” the light reflected back toward the source from the entrance mirror. It is also suggested that if this recycling cavity is short enough, the recycling cavity could be resonance free until one reaches the desired high order sideband, perhaps some THz away. The modulation power in this line would be coupled back toward the source, and could be separated with a Faraday isolator system. It is suggested that such schemes may make it feasible to transfer the stability of one optical source in a phase coherent manner to another source located an appreciable frequency interval away. 
     An article entitled “A Coupled Cavity Monolithic Optical Frequency Comb Generator” by M. Kourogi, T. Enaeni and M . Ohtsu in IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 8, No. 12, December 1996, describes an optical frequency comb generator (or a Fabry Perot electro-optic modulator) that generates ultra short optical pulses, and high order sidebands from a single mode laser input. A high efficiency electro-optic phase modulator is installed in a high finesse optical cavity, and driven with an integer multiple of the cavity free spectral range. 
     Two types of optical frequency comb generators are discussed, each having an external coupled cavity, one to achieve efficient comb generation, and the other to provide a frequency shifter. 
     In the FIG. 1 a  embodiment of this publication, a mirror M 3  was mounted on a PZT transducer, and placed in front of a mirror M 1  to form a coupled cavity, and the coupled cavity was adjusted to the laser frequency. As a result, the incident light is transmitted by the coupled cavity, while the coupled cavity becomes highly reflective for the sidebands generated by the comb generator. 
     To allow the selection of extracted sidebands, the above-described coupled cavity of FIG. 1 a  was removed from the input port of the comb generator, and as shown in FIG. 1 b  of this publication, and PZT mounted mirror M 3  was installed at the output port. By adjusting the bias voltage at the PZT on which mirror M 3  was mounted, an appropriate set of sidebands may be selected. 
     This publication also suggests that if two stable coupled cavities are installed at the input and the output port of the comb generator, the power of the selected sideband may be increased, in which case, the comb generator will become a highly efficiency frequency shifter for a wide frequency range. 
     An article entitled “Efficient optical frequency comb generator” by A. S. Bell, G. M. McFarlane, E. Riis and A. I. Ferguson, OPTICS LETTERS, Vol. 20. No. 12, Jun. 15, 1995, also describes an arrangement having two cavities that are locked to a laser carrier frequency. This publication describes how an unknown laser frequency can be measured with respect to a well-known standard frequency. This publication also describes how large frequency differences can be determined, based on a few rf measurements. A comb of equally-spaced modes is produced from a single laser carrier frequency. An electro-optic modulator superimposes a microwave frequency onto the carrier frequency, thus producing a comb of nodes with spacing of exactly the microwave frequency. The electro-optic modulator is placed into a three mirror dogleg cavity that is resonant to both the carrier frequency and the sidebands. A second cavity is used to ensure that most of the incident laser power is coupled into the optical cavity. To increase the coupling of the laser into the optical cavity, and hence increase the throughput of the comb generator, a PZT-mounted mirror M 1  is placed before the mirror M 2  of the optical cavity that contains the electro-optic modulator to thus form a coupling cavity. The coupling cavity was then frequency locked to the input light. 
     SUMMARY OF THE INVENTION 
     In an implementation of the present invention, an Electro-Optic Modulator (EOM) crystal is placed inside of a low loss, two mirror, comb-generating optical cavity that is in resonance with an input laser carrier frequency, and with all carrier sidebands frequencies. That is, the laser carrier frequency equals an integral multiple of the comb-generating optical cavity&#39;s 
     Free Spectral Range (FSR). Equally important, the radio frequency that is applied to the modulator also is a multiple of the cavity&#39;s FSR. 
     More specifically, an Optical Frequency Comb (OFC) with a span that is wider than 3 THz is provided by a 10.5 GHz resonant EOM modulator that is placed inside of a resonant comb-generating optical cavity that includes two physically spaced mirrors, and whose cavity input is a reference beam produced by a He—Ne laser that operates at about 633 nanometers (i.e., red). A low noise RF microwave oscillator drives the EOM at 10.5-GHz, so that high order sidebands do not quickly collapse due to multiplied phase noise amplitude. 
     A two mirror, thin, bandpass filter optical cavity, having a free spectral range of 2-THz and a finesse of 400, functions as a direct output coupler for the comb-generating cavity. The bandpass filter cavity and the comb-generating cavity share a common fixed position mirror. This bandpass filter cavity is tuned into resonance with the selected sideband of the 633-nanometer laser, thus providing efficient output coupling of a selected sideband power from the comb-generating cavity. At the same time, all other sidebands are kept inside of the resonant comb-generating cavity for continued comb generation. As a result, the bandpass filter optical cavity extracts the full power of a chosen sideband from the OFC, all other sidebands are trapped inside the comb-generating cavity, and the single frequency output ensures a high Signal to Noise Ratio (SNR) in a heterodyne experiment. 
     This invention provides a highly selective Optical Frequency Comb (OFC) generator which can phase coherently bridge a wide frequency interval of more than a few terahertz (THz); for example, 4 THz. The OFC comprises a plurality of equally-spaced spectral lines that are grouped around the reference spectral line established by the carrier frequency laser. The novel features include the provision of an intercavity optical amplifier, such as an optical parametric amplifier, within the comb-generating cavity. 
     It is an object of this invention to provide a comb-generating optical cavity having a first mirror that is physically movable along the cavity&#39;s propagation axis, having a second mirror that is mounted at a fixed position on the propagation axis, and having an optical amplifier and a microwave modulated EOM crystal (for example, Mg:LiNbO 3 ) located on the propagating axis between the first and second mirrors. A reference laser beam (for example, from a He—Ne laser or a Nd:YAG laser) is directed onto the first mirror, and thus into the comb-generating cavity, along the comb-generating cavity&#39;s propagation axis. A direct reflection beam from the first mirror and a leakage beam from the comb-generating cavity are compared to control the position of the first mirror, and to thus control the physical length of the comb-generating cavity. A tunable bandpass filter optical cavity is located on the propagation axis, directly downstream of the comb-generating optical cavity, in a manner to receive the comb output of the comb-generating optical cavity. The bandpass filter optical cavity comprises the second (fixed position) mirror that is within the comb-generating cavity, and a third mirror that is movable along the propagating axis. Selective tuning of the bandpass filter optical cavity is achieved by positioning of this third mirror. In this manner, a selected bandpass characteristic of the bandpass filter optical cavity operates to pass a portion of the comb output. 
     Another object of the invention is to provide apparatus having a comb-generating optical cavity that includes both an optical modulator, for example an electro-optic modulator, and an optical amplifier, for example an optical parametric amplifier. 
     Another object of the invention is to provide apparatus having a comb-generating optical cavity that includes both an optical modulator or electrooptic modulator and an optical amplifier or optical parametric amplifier, and a common mirror bandpass filter optical cavity that directly receives the output of the comb-generating cavity. 
     Another object of the invention is to provide apparatus having a comb-generating optical cavity that includes both an optical modulator or electro-optical modulator and an optical amplifier or optical parametric amplifier, a common mirror output bandpass filter optical cavity, and an input bandpass filter optical cavity that is tuned to the frequency of the input or reference laser. 
     In the various figures discussed below, an optical amplifier or optical parametric amplifier, and an optical modulator or elector-optic modulator are shown as two individual structural elements. As a feature of this invention, these two optical elements may be provided as a single unitary structural assembly rather than two individual assemblies a shown in the various figures. 
     These and other features, advantages and objects of the invention will be apparent from the following detailed description, which description makes reference to the following drawings. 
     The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is useful in understanding the operation of a prior art comb-generating optical cavity. 
     FIG. 2 shows the principle of the present invention in an embodiment wherein a comb-generating optical cavity includes both an optical modulator or an electro-optic modulator, and an optical amplifier or an optical parametric amplifier. 
     FIGS. 3A and 3B present the present invention as shown in FIG. 2, wherein an additional mirror forms an auxiliary, or output bandpass filter cavity to facilitate the efficient recovery of a single output sideband that is within the optical comb, FIG. 3B also showing the optical spectral components involved. 
     FIG. 4 shows an embodiment of the invention wherein an input bandpass filter optical cavity is added to the FIG. 3A embodiment to improve the in-coupling efficiency. 
     FIG. 5 shows an experimental realization of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principle of the present invention can be understood by reference to FIG. 2 which shows an optical comb-generating cavity  71  that includes both an optical amplifier  81  (or an optical parametric amplifier  81 ), and an optical modulator  74  (or an electro-optic modulator  74 ). The order of the two elements  81 ,  17  within cavity  71  is not significant. That is, while optical amplifier  81  is shown positioned upstream of optical modulator  74  (i.e., optical amplifier  81  is closer to laser  69  than is optical modulator  74 ), this relative positioning of optical amplifier  81  and optical modulator  74  is not critical to the invention. 
     In FIG. 2, a comb-generating cavity input mirror  72  and a comb-generating cavity output-mirror  73  are shown as having inward-facing concave surfaces  76  and  77 , and outward-facing flat surfaces  78  and  79 . Using this type of cavity mirror, concave surfaces  76 ,  77  are coated to be highly reflective to the wavelength of laser  69 , and flat surfaces  78 ,  79  contain no coating, or more preferably, are coated to be anti-reflective. Mirror  73  is mounted at a fixed position, and mirror  72  is movable, as will be explained relative to FIG.  5 . That is, mirror  72  is PZT-movable in the manner of mirror  18  of FIG. 5, and mirror  73  is mounted at a fixed-position in the manner of mirror  19  of FIG.  5 . 
     Optical modulator  74  is controlled or modulated by a control system, or network  75 , that comprises RF microwave control input to modulator  74 , or an optical pulse train control input to modulator  74 . More generally, control system  75  operates to vary the optical transmission property of optical modulator  74 , or to vary the effective optical path length of optical modulator  74 . That is, the time of transmission of a beam within optical modulator  74  is controlled or modulated. 
     Operation of the apparatus shown in FIG. 2 provides an optical frequency comb output  90 . Other useful optical configurations to achieve this stable cavity are presented below. 
     As a feature of this invention, optical amplifier  81  and optical modulator  74  may be provided as a single unitary assembly rather than as two individual assemblies as shown in FIG. 2, and the various figures discussed below. 
     An optical amplifier can be based upon any of the many known laser transitions in many laser media. While many laser transitions are known, the number of practically useful laser transitions are relatively few, and generally do not afford full spectral coverage. It is known that non-linear optical processes are a way of shifting a laser wavelength from where it is available to where it is needed. It is also known that the parametric process offers the possibility of generating a continuous range of frequencies from a single frequency source. 
     The term optical parametric amplifier, or more generally optical amplifier as used herein, is intended to mean a frequency difference generator. An optical parametric amplifier implements a second order non-linear optical process, and is a source of broadly tunable coherent radiation that is capable of covering the entire spectral range from the near-UV to the mid-IR with operation down to the femtosecond time domain. 
     The spontaneous parametric process, also known as parametric luminescence or parametric fluorescence, is a process in which an incident photon, called a pump photon, propagating in a non-linear medium, such as a crystal, breaks down spontaneously into two photons of lower frequency; one called a signal photon and the other called an idler photon. 
     In the parametric amplification process, and with only the pump photons present in an initial state, spontaneous emission occurs at frequencies for which the signal and idler frequencies are under phase matched conditions. With signal photons and pump photons present in the initial state, stimulated parametric emission occurs in the same way as in a laser medium, except that the pump photons are converted directly into signal photons, and corresponding idler photons through the second order non-linear optical process, and no exchange of energy with the crystal medium is involved. The parametric amplification process can also be viewed as a repeated difference-frequency process in which the signal photons and idler photons repeatedly mix with the pump photons in the crystal medium, generating more and more signal photons and idler photons under phase matched conditions. 
     It is known that a parametric oscillator can be constructed by adding a pair of Fabry-Perot mirrors to a parametric amplification process that includes a suitable non-linear optical crystal. It is known that optical parametric oscillators are used in wavelength shifting non-linear optical devices. Tuning of the optical parametric oscillator can be achieved by rotating the crystal relative to the direction of propagation of the pump beam or the axis of the Fabry-Perot cavity, or by changing the crystal&#39;s phase-marching conditions with temperature or electric field. 
     An optical modulator, or electro-optic modulator, as used herein is intended to mean a medium, usually a crystal, that, when driven by a control signal, operates to produce optical frequency information carrying sidebands upon an original single frequency beam that is incident upon the optical modulator. The modulator transition phase, or amplitude, may be controlled by a suitable RF or optical control input. 
     For some purposes, it is convenient to receive as an output only a single optical frequency that is within FIG. 2 comb output  90 , wherein this single optical frequency is displaced from the comb-generating optical cavity optical input by a multiple of a control frequency  75 . 
     Such an important version of the present invention is shown in FIGS. 3A and 3B wherein a comb-generating optical cavity includes both an optical amplifier or optical parametric amplifier, and an optical modulator or electro-optic modulator, and wherein a tunable bandpass filter optical cavity is directly coupled to receive a comb output from the comb-generating optical cavity, and wherein the comb-generating optical cavity and bandpass filter optical cavity share a common fixed position mirror. 
     FIG. 3A shows the invention shown in FIG. 2 wherein an additional mirror  87  forms an auxiliary bandpass filter cavity  85  that facilitates the efficient recovery of a single output sideband  86  from comb output  90  of FIG.  2 . FIG. 3B also shows this configuration of the invention, wherein an additional mirror, such as  87  of FIG. 3A, and a common mirror, such as  73  of FIG. 3A, forms an auxiliary bandpass filter cavity  85  that facilitates the efficient recovery of a single output sideband  86  from comb output  80  of comb-generating cavity  16 . 
     More specifically, FIG. 3A shows an embodiment of the invention wherein an output optical bandpass filter cavity  85  (as will be described with reference to FIG. 5 cavity  33 ) is added to the above-described FIG. 2 embodiment. In FIG. 3A, optical bandpass filter cavity  85  is made up of fixed position common mirror  73 , and a third movable mirror  87  that is PZT-movable in the manner of mirror  34  of FIG. 5, as will be described. 
     In this embodiment of the invention, third mirror  87  is shown as having two flat faces  88  and  89 , in which case both faces  88  and  89  contain no coating, or more preferably, flat surfaces  88  and  89  are provided with an anti-reflective coating. The objective of providing focusing action of mirror  73  can be realized with advantage by a convex piano lens, as will be shown below. 
     Optical bandpass filter cavity  85  operates upon the broad frequency comb output  80  of the comb-generating cavity  72 ,  81 ,  74 ,  73  to provide a single comb frequency output  86 . 
     With reference to FIGS. 3A and 3B, reference laser beam  41  is presented to the input mirror  72  of comb-generating cavity  16  and the EOM  74  within comb-generating cavity  16  receives a modulation control signal  22  by way of control network  75 . 
     Mirror  73  that is shared between comb-generating cavity  16  and bandpass filter cavity  85  affects the direct coupling of the two optical cavities  16  and  85 . This common mirror  73 , functioning within comb-generating cavity  16 , operates to provide comb output  80  that comprises comb frequency spectrum  51 , i.e., a large number of sidebands that are centered on the reference frequency  41  of reference laser beam  41 , which sidebands progressively decrease in magnitude away from center frequency  41 . 
     Bandpass filter optical cavity  85 , which includes common mirror  73 , receives a tuning parameter input  52  (i.e., the physical position of mirror  87 ), and as a result of tuning input  52 , optical cavity  85  bandpass operates on comb spectrum  51  to provide a single frequency beam output  86  that comprises a selected one  86  of the many frequencies that are within comb spectrum  51  (or a selected group of the comb frequencies, dependent upon the magnitude of the physical separation between mirrors  73  and  87  that is achieved by tuning parameter  52 , as will be described). 
     The use of a common mirror  73  within both comb-generating cavity  16  and bandpass filter cavity  85  can be appreciated by considering the hypothetical use of two separate cavities, each cavity having two mirrors. In this case, multiple reflections occur between the two, two mirror cavities and must be controlled, for example by the use of an optical isolator that is physically located between the output mirror of the comb-generating cavity and the input mirror of the bandpass filter cavity. The present invention does not require such an optical isolator, and produces a larger output power since in the above four mirror hypothetical case, the desired output comb line is only available due to “leakage” through the fixed mirror of the optical frequency comb generator. Instead, the output of a single comb line  86  can approach the power level that is within the basic comb generating cavity  16 . 
     FIG. 4 shows an embodiment of the invention wherein a tunable input bandpass filter optical cavity  90  is added to the FIG. 3A embodiment to improve the in-coupling efficiency of the comb-generating optical cavity. Input bandpass filter optical cavity  90  is made up of two mirrors that comprise movable mirror  72  that functions within comb-generating optical cavity  72 ,  81 ,  74 ,  73 , and a fourth movable mirror  91 . That is, mirror  72  is common to both comb-generating cavity  72 ,  81 ,  74 ,  73  and tunable input bandpass filter optical cavity  90 . 
     Mirror  91  is shown as having two flat faces  92  and  93 , in which case, both faces  92  and  93  contains no coating, or more preferably, flat surface  92  is provided with an anti-reflective coating. 
     In operation, mirror  91  is moved along the system&#39;s propagation axis, as indicated by arrow  95 , until a mirror position is found where a maximum magnitude is provided in output  86  from bandpass filter optical cavity  85 . This movement of mirror  91  to maximize output  96  corresponds to a minimum reflected light condition and can take place while control source  75  is not operative, in which case, output  86  comprises only the unique frequency of reference laser beam  41 . 
     Input optical cavity  90  operates to increase the efficiency of the system in that input optical cavity  90  provides more efficient coupling of the input beam  41  from laser  69  to comb-generating optical cavity  72 ,  81 ,  74 ,  73 . In this manner, output  86  is generated as in the FIG. 3A,  3 B embodiment, having the large magnitude characteristic that is shown at  86  of FIG.  3 B. However, in the FIG. 4 embodiment, more shifted frequency power exists in output  86  due to the improved input coupling that is provided by input optical cavity  90 . In order to provide a detailed implementation of the above-discussed principles of the invention, FIG.  5  and the following discussion provides a detailed teaching relative to the current best practice of the invention. 
     With reference to FIG. 5 which shows an experimental realization of the invention, a laser  10  generates a reference frequency beam  11  that is directed along a propagation axis  70 , laser  10  being a polarization stabilized laser that provides output beam  11 . While a 150 micro watt He—Ne laser is shown in this embodiment of the invention, the spirit and scope of the invention is not to be limited thereto. In particular, successful experiments have employed a Nd:YAG laser emitting 1.06-micro meter radiation. 
     Beam  11  traverses in order, a first polarized beam splitter (PBS)  12 , a Faraday rotator  13 , a second PBS  14 , and a one-half wave plate  15 . 
     Beam  11 , whose polarization has been modified by directional isolators  12 - 15 , now enters a comb-generating optical cavity that is shown within broken line  16 . Optical cavity  16  includes an optical amplifier  81  or a parametric optical amplifier  81 , and a microwave-driven optical modulator  17 , or a microwave-driven electro-optic modulator  17 . Within the spirit and scope of this invention, EOM  17  comprises any suitable electro-optic crystal. In an embodiment of the invention, EOM  17  comprises a broadband, anti-reflection coated, Mg:LiNbO 3  crystal having dimensions of about 35.4 mm by 1.0 mm by 2.0 mm. 
     Optical amplifier (OA)  81  and electro-optic modulator (EOM)  17  are both embedded within comb-generating optical cavity  16 , this cavity including an entry mirror  18 , and an exit mirror  19 . Mirrors  18  and  19  are identical lens substrates; for example, of a glass or a crystal optical material having an effective focal length of about 25.0 cm. In this embodiment of the invention, the convex faces  20  of mirrors  18  and  19  could be uncoated, or are preferably coated to be anti-reflective (AR) at the working wavelength, and the flat faces  21  of these two mirrors are coated to have high reflectivity, for example about 99.6%. 
     EOM  17  is driven by an AC microwave source  22  having a frequency of about 10.5-GHz. The design of the microwave EOM  17  uses a waveguide geometry that forces a match in OEM crystal  17  between the microwave phase velocity and the optical phase velocity of the laser beam  11  that is emitted by laser  10 . The microwave resonance at 10.5-GHz has a bandwidth of about 0.3-GHz and a Q factor of about 230. A modulation index of about 0.8 is obtained with a microwave power of about 0.6 watts. 
     In order to lock the propagation length of comb-generating optical cavity  16  onto the wavelength of input laser  10 , the position of input or entry mirror  18  is controlled by operation of a piezoelectric transducer (PZT)  23 . As will be appreciated by those of skill in the art, two beam portions R1 and R2 are “reflected” by comb-generating optical cavity  16 . The first of these two beam components,  25  or R1, comprises a beam that is directly reflected from the flat surface  21  of input mirror  18 , whereas the second of these two beam components,  26  or R2, comprises a cavity modulated beam that leaks out from comb-generating optical cavity  16 . A composite beam  27 , comprising R1+R2, is detected by a photodiode  28 . The electrical output  29  of photodiode  28  is provided as an input to cavity lock network  30 . Cavity lock network  30  operates to provide an electrical output  31  that energizes PZT  23 , to thus cause input mirror  18  to physically move as shown by arrow  32 . In this way, operation of cavity lock network  30  compares the phase and magnitude of directly reflected beam  25  to the phase and magnitude of leakage beam  26 , and generates an output  31  that is effective to move mirror  18  so that the quantity R1+R2 is minimized. In an embodiment of the invention, the beam minimizing physical position of mirror  18  was dithered by applying a dither frequency signal  32  to cavity lock network  30 , the dither length being relatively small (about {fraction (1/10)} th  of the cavity line width). Light beam  27 , or R1+R2, was then phase sensitive detected by cavity lock network  30  against dither frequency  32  to provide a cavity discriminator signal. 
     In FIG. 5, a physically short dimension output, and tunable bandpass filter optical cavity, identified by broken lines  33 , is made up of above-mentioned fixed position mirror  19  and a third mirror  34 . Mirror  34  is identical to mirrors  18  and  19  in that it preferably has an identical lens substrate with an effective focal length of about 25 cm, a convex face  35  that is coated to be anti-reflective at the working wavelength, and a flat face  36  that is coated to have high reflectivity; for example, about 99.6% reflective. 
     In order to selectively tune bandpass filter optical cavity  33 , mirror  34  is mounted onto a slide stage  37  that is movable, for example manually, in the propagation direction as indicated by arrow  38 . Movement of slide stage  37 , as affected by a micrometer, operates to adjust the very short physical separation  40  that exists between the flat surface  21  of mirror  19  and the flat surface  36  of mirror  34 . This bandpass tuning of filter cavity  33  operates to cause a desired one, or a group of, the large number of comb frequencies that are present at beam location  39 , between mirror  19  and mirror  34 , to be provided as an output beam  41  from bandpass filter optical cavity  33 . 
     By way of example, distance  40  is adjusted to be in the micron range when a single comb frequency  41  is desired, and distance  40  is adjusted to be in the millimeter range when a group of comb frequencies are desired at bandpass filter output  41 . Coarse tuning of optical cavity  33  is produced by micrometer adjustment of slide stage  37 , whereas fine tuning of optical cavity  33  is produced by operation of PZT  42 . 
     As a feature of the invention, PZT  42  is mounted on the precision mechanism or motion stage  37  to allow spacing  40  to be varied from micro meters to a few millimeters. PZT  42  is energized so as to fine tune bandpass filter optical cavity  33  to a selected comb frequency or frequencies. 
     In an embodiment of the invention, the comb-generating cavity  16  that is formed by mirrors  18  and  19  has a finesse of about 400, a FSR of about 2 THz, a transmission efficiency of about 30%, and increased output power of the selected sideband  41  by a factor of 150. 
     In FIG. 5, the portion  43  of the OFC&#39;s comb output  41  is monitored by a DC photodetector in the form of photodiode  44 , and the portion  45  of the OFC output  41  is sent to an avalanche diode  46  for heterodyne mixing with the output  47  of a cavity external, and tunable laser diode  48  that is tuned by operation of spectrascope and wavemeter  49 . Avalanche diode  46  operates to provide heterodyne detection of the selected OFC sideband  41 . 
     In an embodiment of the invention, operation of the apparatus of FIG. 5 provides enough resolution to resolve individual comb sidebands that are spaced about 10.5-GHz apart, and good SNRs were observed beyond sideband number  150 . For a still wider comb output, a wider FSR of the comb-generating cavity  16  or comb-line-selecting cavity  33 , will be appropriate. 
     In FIG. 5, the efficiency of the OFC generator is improved by the use of the two mirrors  19 ,  34  that make up a short filter cavity filter  33  that operates to permit the efficient escape of the selected comb sideband component(s)  41 . With limited power from He—Ne laser  10 , a beat signal  50  with a SNR of 20 dB and a 100 kHz bandwidth was produced. 
     As stated above, motion stage  37  and PZT  42  are used to position mirror  34  along propagation axis  70  in order to provide a peak or power output for a desired comb frequency(s) component  41  that is within comb output  39 . While a number of tuning schemes will be apparent to those of skill in the art, one scheme involves turning off microwave source  22 , such that the only frequency now within output  39  is that of reference frequency of laser  10 . The position of mirror  34  is then adjusted to seek a maximum signal output  41 . As a result, it is known that bandpass filter optical cavity  33  is now tuned to this reference frequency, and it is also known that this reference frequency is the center frequency of any subsequently-generated comb output  39 . The gap dimension  40  that now exists between mirrors  19  and  34  can be called a reference gap. 
     Microwave source  22  is now turned on, and a multi-frequency comb output  39  is now generated. It is known that the comb frequencies are spaced by the 10.5-GHz frequency of microwave source  22 . Thus, a desired one of the frequencies within comb output  39  can now be found by moving mirror  34  and counting a precalculated number of maximum/minimum signal intensities in output  41 , whereupon it is known that the desired sideband frequency is now being passed by bandpass filter optical cavity  33 . 
     It is also known that when a desired sideband frequency  41  is higher than reference frequency  11 , reference gap  40  between mirror  19  and  34  must be decreased in size in order to count and thus find that higher frequency sideband, and it is known that when a desired sideband frequency  41  is lower than reference frequency  11 , reference gap  40  between mirror  19  and  34  must be increased in size in order to count and thus find that lower frequency sideband. 
     In embodiments of the invention, but without limitation thereto, an optical parametric amplifier utilized was a MgO:LiNbO 3  crystal heated to about 108-degrees centigrade, to provide phase matching at 1064 nanometers when pumped by a CW 532 nanometer pumping beam. 
     The invention has been described in detail while making reference to various embodiments thereof. Since it is known that others skilled in the art will readily visualize yet other embodiments that are within the spirit and scope of this invention, the above detailed description is not to be taken as a limitation on the spirit and scope of this invention.