Abstract:
A compact solid-state laser has its cavity formed by the reflection surface of a gain crystal and the reflection surface of a chirped mirror. Pumping light is incident to the cavity through the reflection surface of the gain crystal or the chirped mirror, from which the laser output is led out. Forming of the cavity solely by the gain crystal and chirped mirror enables the elimination of additional component parts for the compensation of dispersion, thereby making the entire laser system compact, and consequently permitting an increase in the repetition frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/997,700 filed on Dec. 23, 1997 now U.S. Pat. No. 6,091,495, which is now pending to USPTO and the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a compact solid-state laser, and particularly to a compact solid-state laser used for a multiwavelength optical source for a wavelength-division-multiplexed communication system, a wide-spectrum optical source for spectroscopy, a ultra-short pulse source for distance measurement, a ultra-short pulse source for instrumentation, and the like. The invention also relates to a transmitter using this compact solid-state laser. 
     Various cavity structures for the solid-state laser have been proposed, as described for example in publication “Ultra Short Pulse Technology” (“KOGAKU” Japanese Journal of Optics, Vol.24, No.7, pp.378). Their basic structure includes four or more mirrors which form a cavity, a gain crystal as gain medium, and a prism pair or a grating pair which compensates the dispersion attributable to the gain crystal. 
     However, this basic structure has too many component parts and needs to space the prism pair or grating pair by several tens centimeters for dispersion compensation. Therefore, this structure it does not allow for the compact design of a solid-state laser. In the case of pulsative operation, the repetition frequency cannot be raised unless the cavity length is made short. The conventional solid-state laser has a repetition frequency of around 100 MHz, whereas the solid-state laser used for the communication system must have a repetition frequency of several gigahertz or higher. 
     There is a proposal for making the solid-state laser compact so that its repetition frequency is higher, as described in the article entitled “Compact Kerr-lens model-locked resonator” by B. E. Bouuma et al. in OPTICS LETTERS, Vol.21, No.2, pp.134-136, published in 1996. The proposed compact Kerr-lens model-locked resonator has its cavity made up of three elements including a gain crystal, a curved mirror, and a prismatic output coupler. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to make the solid-state laser compact so that its repetition frequency is several gigahertz or higher and it is usable for the communication system. 
     For making a compact solid-state laser, it is necessary to reduce the number of component parts and compensate the dispersion attributable to the gain crystal without using the prism pair or grating pair. The above-mentioned proposal by B. E. Bouuma et al. uses a prism and a crystal pair, instead of using a prism pair, for dispersion compensation, thereby reducing the number of cavity elements to three so as to provide a compact solid-state laser. 
     In contrast, the present invention is intended to offer a compact solid-state laser which is designed to compensate the dispersion by means of a chirped mirror formed by multilayer coating (described in the article entitled “Chirped multilayer coating for broadband dispersion control in femtsecond laser” by Robert Szipocs et al. in OPTICS LETTERS, Vol.19, No.3, pp.201-203, published in 1994), so that the cavity is virtually formed of only two elements of a chirped mirror and a gain crystal. Specifically, the gain crystal has its one side rendered with a reflection coating, chirped-mirror coating, or saturable-absorber mirror coating so that it functions as a mirror, with the chirped mirror serving as another mirror of the cavity, thereby reducing the number of mirrors required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the arrangement of a compact solid-state laser based on an embodiment of this invention; 
     FIG. 2 is a diagram showing the arrangement of a compact solid-state laser based on another embodiment of this invention; 
     FIG. 3 is a diagram showing the arrangement of a compact solid-state laser based on still another embodiment of this invention; 
     FIG. 4 is a diagram showing the arrangement of a compact solid-state laser based on still another embodiment of this invention; 
     FIG. 5 is a diagram showing an embodiment of the gain crystal which can be used for the inventive compact solid-state laser; 
     FIG. 6 is a diagram showing another embodiment of the gain crystal which can be used for the inventive compact solid-state laser; 
     FIG. 7 is a diagram showing still another embodiment of the gain crystal which can be used for the inventive compact solid-state laser; 
     FIG. 8 is a diagram showing an embodiment of the chirped mirror which can be used for the inventive compact solid-state laser; 
     FIG. 9 is a diagram showing another embodiment of the chirped mirror which can be used for the inventive compact solid-state laser; 
     FIG. 10 is a diagram showing the arrangement of a compact solid-state laser in which a Brewster-cut gain crystal is used and the lens in optical source is tilted for astigmatism compensation. 
     FIG. 11 is a diagram showing the arrangement of a compact solid-state laser in which a glass or fused-silica plate is set to break rotational symmetry with respect to optical beam and to improve polarization characteristics. 
     FIGS. 12A and 12B are a plan view and side view of a transmitter of the wavelength-division-multiplexed communication system, with the inventive compact solid-state laser being applied thereto; and 
     FIG. 13 is a perspective view of the compact solid-state laser used in the transmitter shown in FIG.  10 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of this invention will be explained with reference to the drawings. 
     FIG.  1  through FIG. 4 show the arrangements of compact solid-state laser based on the embodiments of this invention. The compact solid-state laser has its cavity formed between the right-side reflection surface la of a gain crystal  1  and the right-side reflection surface  2 a of a chirped mirror  2 . Reference numeral  6  denotes the pumping light directed to the gain crystal  1 ,  3  is the optical source of the pumping light,  4  is the laser output, and  5  is a dichroic mirror which diverts the course of the laser output  4 . 
     The compact solid-state laser can have several arrangements depending on the manner of leading in the pumping light  6  and leading out the laser output  4 , of which four representative types are shown in FIG.  1  through FIG.  4 . 
     In the arrangement of FIG. 1, the pumping light  6  is incident to the chirped mirror  2  from its back, and the laser output  4  is led out from the reflection surface la of the gain crystal  1 . In the arrangement of FIG. 2, the pumping light  6  is incident to the reflection surface  1   a  of the gain crystal  1 , and the laser output  4  is led out from the back of the chirped mirror  2 . In the arrangement of FIG. 3, the pumping light  6  is introduced into the cavity by being incident to the back of the chirped mirror  2  through the dichroic mirror  5 , and the laser output  4  is taken out from the back of the chirped mirror  2  and led out by being diverted by the dichroic mirror  5 . In the arrangement of FIG. 4, the pumping light  6  and the laser output  4  exist on the side of the right-side reflection surface la. In the arrangement of FIG. 4, the pumping light  6  is introduced into the cavity by being incident to the reflection surface  1   a  of the gain crystal  1  through the dichroic mirror  5 , and the laser output  4  is taken out from the reflection surface la of the gain crystal  1  and led out by being diverted by the dichroic mirror  5 . 
     The gain crystal  1  used in the foregoing arrangements of solid-state laser can have various structures as shown in FIG. 5, FIG.  6  and FIG.  7 . In any case, the gain crystal  1  has its right-side reflection surface la rendered with a reflection coating, chirped mirror coating, or saturable-absorber mirror coating. For leading out the laser output  4  from the reflection surface la of the gain crystal  1 , as in the cases of FIG.  1  and FIG. 4, the reflection surface  1   a  is coated to become an output-coupler mirror of several percent or less. For leading out the laser output  4  from the back of the chirped mirror  2 , as in the cases of FIG.  2  and FIG. 3, the gain crystal  1  has its reflection surface la rendered the high-reflection coating. 
     The gain crystal  1  has its left-side surface  1   b  rendered the anti-reflection coating, and further may be rendered with a saturable-absorber coating, instead of the saturable-absorber mirror coating of the reflection surface  1   a,  as shown in FIG.  5 . Or, the gain crystal  1  is polished to provide a Brewster angle θ B  so as to suppress the reflection, as shown in FIG.  6  and FIG.  7 . Shown in FIG. 6 is the simplest structure of gain crystal having a Brewster angle θ B . with its reflection surface la being rendered with a planar cutting. 
     However, the Brewster angle θ B  differs slightly depending on the wavelength. Accordingly, in dealing with a wide spectrum as in the case of ultra-short pulse oscillator, the optical path varies in the gain crystal  1 . It can be correct effectively by the provision of a cylindrical curving surface for the reflection surface la as shown in FIG.  7 . Shown by the dashed line  6  in FIG.  6  and FIG. 7 is the incident pumping light  6  of the case of incidence on the side of the reflection surface la. 
     FIG.  8  and FIG. 9 show examples of the structure of chirped mirror  2  which can be used in the foregoing arrangements of solid-state laser. The chirped mirror  2  is a concave mirror having a focal distance of around several tens millimeters, which is determined depending on the repetition frequency. The chirped mirror has a multilayer coating on the right-side surface  2 a as shown in FIG.  8  and FIG.  9 . 
     In the arrangement of FIG. 4, where the pumping light  6  and laser output  4  are led in and out on the side of the reflection surface  1   a  of the gain crystal  1 , the chirped mirror  2  may have its unused back surface (opposite to its mirror surface  2   a ) simply left planar as shown in FIG.  8 . 
     On the other hand, in the arrangements of FIG. 1, FIG.  2  and FIG. 3, where the pumping light  6  or laser output  4  is led in or out on the side of the chirped mirror  2  opposite to its mirror surface  2   a,  the respective optical systems are required on that side. In the arrangement of FIG. 1, where only the pumping light  6  is incident to the back surface of the chirped mirror  2  opposite to its mirror surface  2   a,  the mirror  2  may have its back surface simply left planar as shown in FIG.  8 . In the arrangement of FIG.  2  and FIG. 3, where the laser output  4  is focused, the chirped mirror  2  may be designed to have its solid body working as a focusing lens so that the whole device becomes much smaller. 
     In addition, in the arrangements of FIG. 1, FIG.  2  and FIG. 3, where the pumping light  6  or laser output  4  is led in or out on the side of the chirped mirror  2  opposite to its mirror surface  2   a,  it is effective to provide a anti-reflection coating on this surface. 
     The pumping optical source  3  is a solid-state laser or a semiconductor laser, the latter being useful for the compactness of the whole device. The pumping optical source  3  may include a lens system for the enhancement of pumping ability, and may include an isolator, also. 
     The pumping optical source  3 , gain crystal  1  and chirped mirror  2  have a common optical axis obviously, as will be explained in regard to the structure in connection with the transmitter of optical communication system. In case that gain crystal  1  is a Brewster-cut one like FIG. 6 or FIG. 7, and that pumping light  6  is introduced into the cavity through the chirped mirror  2  like FIG. 1 or FIG. 3, then the lens  32  in optical source  30  is tilted by a few or more degrees to compensate for astigmatism, as shown in FIG. 10, where an isolator  33  is also drawn. 
     When gain crystal  1  is a simple plane parallel structure as shown in FIG. 5, then the cavity has rotational symmetry with respect to optical beam, and the light is not polarized, leading to poor laser characteristics. To circumvent this problem, a glass or fused-silica plate  34  may be set in the cavity at a Brewster angle q B , as shown in FIG.  11 . 
     Next, an embodiment of the transmitter, with the foregoing compact solid-state laser being applied thereto, will be explained. 
     Among the materials useful for the gain crystal  1 , which include Cr-doped YAG (Cr:YAG crystal), Ti-doped A 1   2 O 3 , Cr-doped LiSAF, Cr-doped Mg 2 SiO 4 , Nd-doped glass etc., the compact solid-state laser used in this transmitter adopts the Cr:YAG crystal having an oscillation wavelength of the 1.5-μm band. The absorption band of the Cr:YAG crystal has a peak at a wavelength around 1.05 μm. 
     For the pumping optical source  3 , a solid-state laser of Nd:YVO 4 , Nd:YLF or Nd:YAG, or a semiconductor laser can be used. The Cr:YAG laser has a wide spectrum (200 nm at maximum) when it is operated in ultra-short pulse oscillation, and accordingly it is useful for the optical source of the wavelength-division-multiplexed communication system. 
     FIGS. 12A and 12B show the plan view and side view of a transmitter of wavelength-division-multiplexed communication system which employs a compact solid-state laser of Cr:YAG. Indicated by numeral  4000  is the Cr:YAG laser, which consists of a pumping optical source  3 , a gain crystal  1  and a chirped mirror  2 , as has been shown in FIG. 2, and it provides all wavelengths necessary for the wavelength-division-multiplexed communication system. Indicated by  1000  is a signal synthesizer, which converts the light, which comes from the Cr:YAG laser  4000  and is incident to the optical fiber  101 , into wavelength-division-multiplexed signals in response to the signals to be transmitted. Indicated by  3000  is a constant-temperature maintaining device, which maintains the suitable temperature for the operation of the signal synthesizer  1000  and Cr:YAG laser  4000  and also serves for the chassis of these parts. 
     The signal synthesizer  1000  can be the one that has been offered in the U.S. patent application Ser. No. 08/997,700 (and the corresponding EP patent publication EP 0 851 205) which is the preceding patent application of the present application, and it will be explained here only briefly. 
     The incident light from Cr:YAG laser  4000  is introduced into a waveguide  101 . The incident light is introduced into an optical path constituting the interferometer  100  through the waveguide  101 . Since the other waveguide  102  contacts the waveguide  101  so as to form a separation path in the interferometer  100 , the incident light is divided into two optical-paths corresponding to the waveguides  101  and  102 . These two optical-paths are brought into contact with each other again at positions where their optical path lengths differ from each other. As a result, interference occurs in the above-described incident light and hence waveforms having predetermined spectrum are obtained from exits of the optical paths  101  and  102 . The light emitted from the optical path  101  and the light emitted from the optical path  102  are introduced into the multichannel modulators  200  and  300 , respectively. The multichannel modulators  200  and  300  are substantially identical in configuration to each other. The light or optical pulses launched from the optical paths  101  and  102  to the multichannel modulators  200  and  300  are set to parallel light beams by cut-away portions  201  and  301  having plano-concave shapes acting plano-convex lenses, respectively. The parallel light beams transmitted through the cut-away portions  201  and  301  are respectively introduced to cut-away portions  211  and  311  serving as diffraction gratings. The light or optical pulses separated into every frequency by the diffraction gratings  211  and  311  are focused on the spatial light modulators  231  and  331  every frequency through cut-away portions  221  an  321  acting plano-convex lenses. The spatial light modulators  231  and  331  allow the light or optical pulses subjected to optical modulation every frequency to pass therethrough according to the modulating signals S 1  and S 2  because voltages for varying absorptance or refractive index are respectively applied to focusing positions associated with each frequency. The optical pulses transmitted through the spatial light modulators  231  and  331  are respectively introduced to cut-away portions  251  and  351  used to serve as diffraction gratings through cut-away portions  241  and  341  functioning as plano-convex lenses, where they are restored to the parallel light beams. The optical pulses restored to the parallel light beams are focused on their corresponding optical paths  103  and  104  through cut-away portions  261  and  361  acting plano-convex lenses. The optical pulses introduced into the optical paths  103  and  104  are coupled by the optical coupler  400  so that a signal waveform for sending is obtained. In FIG. 12A, signal lines for the spatial light modulators  231  and  331  are omitted. 
     The output signal is transmitted in an arbitrary manner to a receiving terminal, in which the signal is demultiplexed by means of a proper demultiplexer and detected in terms of individual wavelengths by means of proper detectors. 
     FIG. 13 illustrates the compact solid-state laser  4000  shown in FIG.  12 . Attached on a base plate  400  having a small coefficient of thermal expansion are part holders  50  each consisting of a holder  51  for mounting the pumping optical source  3 , gain crystal  1  or chirped mirror  2 , and a fixture  52  which serves to fix the holder  50  to the base plate  400 . A pair of vertical angle adjusting means  53  and horizontal angle adjusting means  54  are equipped between the holder  51  and fixture  52 . After the parts  3 ,  1  and  2  are mounted on the holders  51  and the fixtures  52  are fixed to the base plate  400 , the angle adjusting means  53  and  54  are operated so that these parts have a common optical axis as shown by the dash-dot line. 
     The part holders  50  and associated angle adjusting means  53  and  54 , which are shown very simply in the figure, are specifically commercially-available holders named “CENTER MOUNT”, Models 9807,9813M &amp; 9855 manufactured by ν-Focus Corp., or holders named “ULTIMA Series” manufactured by Newport Corp., for example. 
     In this embodiment, the pumping optical source  3  is made up of a semiconductor laser  31  and a convergent lens  32 . The pumping optical source  3  may also include an isolator between the convergent lens  32  and the gain crystal  1  although it is not shown in FIG.  13 . The gain crystal  1  produces much heat which cannot be conducted sufficiently to the constant-temperature maintaining device  3000  through the part holder  50  and base plate  400 , and therefore there are attached Peltier&#39;s elements  61  on both side walls of the holder  51  of the gain crystal  1 . Another Peltier&#39;s element  63  is attached to the pumping optical source  3  in its section close to the semiconductor laser  31 , so that the laser  31  is cooled and the tuning of wavelength based on temperature control is made possible. The wiring of the Peltier&#39;s elements are not shown in the figure. 
     According to this embodiment, the transmitter can be made compact based on the compact solid-state laser and signal synthesizer. 
     The conventional solid-state laser is not small enough, and therefore it is virtually limited to experimental uses by specialists. 
     Many of the solid-state laser have wide gain bandwidths and are operative in ultra-short pulsation, and have superior characteristics that are missing in the semiconductor laser. The inventive compact solid-state laser is capable of being built in a variety of measuring instruments. 
     In the conventional wavelength-division-multiplexed communication system, multiple laser devices are arrayed to arrange optical sources, whereas, using such a solid-state laser as a Cr:YAG laser enables the generation of multiple wavelengths from a single optical source. 
     The inventive compact solid-state laser which is operative at a higher repetition frequency can readily be incorporated in the wavelength-division-multiplexed communication system, and will significantly the transmission capacity of wavelength-division-multiplexed communication system.