End and Side Pumped Laser

The present invention relates to lasers which are optically pumped with semiconductor diode lasers and further relates to lasers with output beam modulation.

A solid state laser, such as a Nd:YAG laser, is typically optically pumped and may be pumped by one or more semiconductor diode lasers or laser arrays. One pumping configuration, called a "side pumped laser" configuration, is disclosed in U.S. Pat. No. 4,575,854 to Martin. The patent describes a laser with a centrally located Nd:YAG rod and a plurality of diode bar arrays arranged circumferentially around the rod. Not all arrays are driven at the same time, but instead are switched between sequential bars, so that the arrays may operate uncooled with a low duty cycle and the rod output is a continuous wave. An advantage of this configuration is that the large surface area on the side of laser rod enables use of high power laser diode arrays resulting in high power output from the laser rod. The large amount of space around the side of the rod also enables thermoelectric coolers and heat sinks to be used so as to maintain the pump light wavelength from the laser arrays close to an absorption band of the rod.

However, in a side pumped laser configuration the rod volume is unhomogeneously pumped. Pumping is very strong near the rod surface where the light is injected so the pump volume is not well confined to the deiired TEM.sub.00 cavity mode. As a result, higher order transverse modes lase and the output may exhibit spikes due to gain switching, especially at the beginning of a lasing pulse (before a steady state is attained). Attempts to maintain single TEM.sub.00 mode operation by expanding the mode volume, for example, can result in a great increase in laser threshold and lower conversion efficiency.

A second pumping configuration, called an "end pumped laser" configuration, is disclosed in U.S. Pat. No. 4,653,056 to Baer et al. That patent describes a laser with a Nd:YAG laser rod together with a laser diode array toward the rear. A collimating lens converts the diverging beam from the laser diode array into a substantially parallel beam, then a lens focuses the beam into the back end of the rod. End pumped lasers are very efficient due to the full confinement of the pumped volume to the desired TEM.sub.00 mode. However, laser diode arrays have a very high aspect ratio. The emitting region is thin, on the order of 1 .mu.m thick, in a direction perpendicular to the active region and can be extended to a width of several millimeters or more in a direction parallel to the active region to increase pump power. It is difficult to end couple the highly asymmetric light from the laser array into the desired TEM.sub.00 cavity mode of the rod. The use of coupling optics and expansion of the cavity mode volume overcomes this problem for laser array widths only up to several hundred microns. Accordingly, the end pumped laser's output power is limited.

For communication and other applications it is desirable that a laser be modulated at a high rate with well controlled pulses. Presently, solid state lasers are modulated in various ways using acousto-optics, electro-optics or other effects. To obtain short pulses, Q-switch or cavity dumping techniques are used which typically also employ an electro-optic or acousto-optic cell to control the output pulses. However, these techniques require extra elements in the laser. Low frequency pulsed modulation can also be achieved by using pulsed lamp pumping. The repetition rates of this technique are usually less than 100 Hz. Semiconductor diode lasers, and diode laser arrays can also be modulated at high rates near threshold by varying the injection current used to pump these devices. However, modulating a solid state laser by means of diode laser array optical pump sources has been generally unsuccessful due to delays between optical pumping and laser output on the order of 10 microseconds or longer which limits the obtainable modulation frequency to tens of kilohertz. An additional problem with side pumped lasers is the spiking due to gain switching, which limits their utility since optical communications and other applications require stable pulses in the TEM.sub.00 mode.

It is an object of the present invention to produce a laser with high power output, a stable transverse mode, and suppressed gain switching.

It is another object of the present invention to produce a laser which can be modulated at a high rate with no delay and little or no spiking, and which allows many types of modulation codes to be used.

The above objects have been met with a laser that is optically pumped using a combination of end and side semiconductor laser sources. An active medium in a resonant optical cavity, such as a solid state laser rod disposed between a pair of reflectors, is optically pumped to produce a coherent light output. A first optical pumping source directs radiant energy into an end of the active medium along its axis. This first source can be a relatively low power LED, laser diode or laser diode array operating in either a cottinuous wave or pseudo-continuous, i.e. high duty cycle pulsed, mode. First source may also comprise multiple light sources directed into the end of the active medium. A second optical pumping source directs radiant energy into a side of the active medium. This second source can be one or more high power laser diode array operating in either high power pulses or modulated at a high rate. Such diode laers can be directly modulated up to gigahertz rates.

The first optical pumping source, i.e. the end pumps, pumps a volume of the active medium which coincides with a desired transveres cavity mode, such as the TEM.sub.00 mode. Pumping occurs at a rate which is sufficient to achieve lasing operation in the desired mode. The second optical pumping source, i.e. the side pump, need not be confined to the desired mode volume. Since the laser is already operating in the desired mode due to the first pumping source, the second pumping source has the effect of amplifying the intensity of the light output in the established desired mode. When the second pumping source is modulated, it is found that the laser light output is the amplified modulated input superimposed on the continuous wave TEM.sub.00 mode light. The solid state laser output follows the modulated pump diode laser up to about 1 MHz or higher depending on the laser cavity length, mirror reflectiveness, internal losses, the material spontaneous and stimulated lifetimes and the power level of end pumping. Still higher modulation rates up to that of the diode laser itself may be possible given the proper material and laser parameters. This occurs with insignificant optical pulse delay and suppressed optical spiking. These features allow many types of modulation schemes to be used, including amplitude modulation, pulse width modulation, frequency modulation, pulse position modulation and the like.

FIG. 1 is a simplified side view of a laser of the present invention.

FIG. 2 is an expanded top view of an end pumped portion of the laser in FIG. 1.

FIG. 3 is a side view of the end pumped portion of FIG. 2.

FIG. 4 is a graph comparing side pump power output and laser power output versus time for a laser of the prior art.

FIG. 5 is a graph comparing side pump power output and laser power output versus time for the laser of FIG. 1.

FIG. 6 is a table comparing laser output powers for various end and side pumping conditions.

With reference to FIG. 1, a laser of the present invention includes an active medium 10, a first optical pumping source 12 and a second optical pumping source 14. Active medium 10 is typically a solid state laser rod. Such rods may be composed of material in either a crystalline or glass host material including Nd:YAG, Nd:glass, Nd:YLF, Nd:LLGG, ruby, alexandrite and other materials commonly used in solid state lasers. In addition to rods, slabs and other active medium geometries can be used. Alternatively, active medium 10 can be a liquid, such as a chelate or dye, or an optically pumpable gas. Active medium 10 is disposed in a resonant optical cavity defined between reflectors 16 and 18. For an active medium 10 which is a Nd:YAG rod, reflector 16 is typically a dielectric end coating on the rod which is highly reflective at 1.06 .mu.m, the wavelength of the laser output, and antireflective at 0.81 .mu.m, the wavelength of the pump radiation corresponding to an absorption band of the Nd:YAG rod. A typical high reflectivity dielectric stack coating has a reflectivity of about 99.8% at 1.06 .mu.m and a transmissivity of about 93% at 0.81 .mu.m. An antireflection coating 20 at 1.06 .mu.must be disposed on the opposite end of active medium 10. Reflector 18 may be a 95% reflecting (at 1.06 .mu.m) concave mirror with a 0.1 to 2 meter radius of curvature mounted approximately 5 to 10 cm from planar reflector 16. Alternatively, reflector 18 may be a mirror which is fabricated directly on the output end of the active medium. The resulting beam waist radius in the resonant cavity is approximately 130 .mu.m to 400 .mu.m for the TEM.sub.00 mode and positioned near reflector 16. Alternatively, the spacing and curvatures of reflectors 16 and 18 may be adjusted to expand or contract the TEM.sub.00 mode waist to 400 .mu.m or more, or 130 .mu.m or less. An antireflective coating may be placed along the rod or slab solid state laser in the side region where the second diode laser pump light is incident. This increases the amount of pump light entering the solid state laser. In addition a reflecting coating or an external reflecting surface can be used to allow this pump light to make multiple passes in the solid state medium prior to absorption.

First optical pumping source 12 is typically a relatively low power laser diode array operating in a continuous wave or quasi-continuous wave mode and emitting light 24 with a wavelength near an absorption band of the active medium 10. A thermoelectric cooler or heat sink 22 may be used to heat or cool the laser array so as to match the laser array's output wavelength to the absorption band of active medium 10. A typical laser diode array for end pumping a Nd:YAG rod laser is a 0.5 W cw, 0.81 .mu.m laser array, model no. SDL-2430-C, commercially available from Spectra Diode Laboratories of San Jose, Calif. Though rated at 0.5 W, up to 2.0 W cw output has been demonstrated. Laser diode array 2 emits light beam 24 which is directed by coupling optics 26 into an end 16 of active medium 10. One optical system which may be used for coupling optics 26 is shown in FIGS. 2 and 3. Optics 26 takes the asymmetric light beam 24 from laser diode array 12, having a lateral divergence .alpha. and a relatively greater transverse divergence .theta., and converts it into a more symmetric focused beam having a lateral convergence .alpha. and a transverse convergence .theta./4. A typical optical system for use with the present invention comprises a Melles-Griot 6.5 mm 0.615 NA collection lens 28 that substantially collimates the beam, a 4.times. anamorphic prism pair 30 and 32 that expands the lateral beam width, and a 25.6 mm focusing lens 34 that focuses the beam 24 into end 16 of active medium 10. The values given above are merely exemplary and other optical systems may also be used. Alternatively, light from one or more LEDs, laser diodes or laser diode arrays 12 may be coupled into end 16 of active medium 10 via one or more light transmissive fibers. Other coupling optics which may be used include other mirror or lens coupled optics, fiber bundles, fiber bundles with mixing rods, and holographic optics.

Second optical pumping source 14 is typically a high power wide area laser diode, laser diode array or fiber coupled laser diode. Alternatively, stacked laser diode arrays, any laser diode, LED, arc lamp or other optical pump could be used as the second optcial pump source. With the rapid advances in semiconductor laser diode technology there are now single linear array devices commercially available with pulsed outputs over 25 watts per centimeter. A typical laser diode array 14 for use with the present invention is a 1 cm wide array emitting from 25 to 135 Watts peak power at about 0.81 .mu.m wavelength for a pulse width of about 150 to 200 .mu.sec and a repetition rate of 100 Hz or with shorter pulses and higher repetition rate, up to many tens of KHz. A thermoelectric cooler 36 may be used to maintain the temperature of laser diode array 14 so as to produce an output at an absorption band of the active medium 10. A single laser diode array 14 close coupled to a side 38 of active medium 10 may be used, but multiple laser diode arrays, single diode laser, fiber coupled diode lasers, or lens or mirror coupled diode lasers may also be used.

In operation, laser diode array 12 pumps a desired volume of active medium 10 from an end in order to achieve steady state continuous wave lasing in a desired transverse cavity mode typically the TEM.sub.00 mode. The desired volume typically coincides with a desired transverse cavity mode, the end pump source 12 thereby preferentially pumping said mode. The coherent light output of the combination end and side pumped laser is thereby established in the desired mode. Laser diode array 12 preferably operates in a continuous wave mode with an output power of at least the pumping threshold of active medium 10 for the desired mode, but may also operate in a quasi-continuous (high duty factor) pulsed mode. Pulse widths on the order of 150 milliseconds are typical, although shorter high repetition rate high peak power pulses may also be used. The second laser diode array or light source 14 pumps active medium 10 from the side, the side pump filling a substantial portion of the volume of the active medium. The high power side pumping pulses temporarily increase the gain at different locations of the active medium. Since the laser is already operating above threshold due to the end pumping, the existing electric field causes depletion of the inverted population in the pulsed pumped volumes confined to the defined cavity mode set by the end pumped source. Thus the continuous wave electrical field prevents the development of high amplitude gain switched spikes, relaxation oscillation, unstable output patterns or other undesirable effects when the side pump is activated. The laser can be operated either near threshold or well above threshold depending upon the intensity of side pump 14, the side pump amplifying the intensity of the coherent laser output in the desired mode. Alternatively, the laser can be operated with a multimode output by not preferentially pumping the TEM.sub.00 mode.

With reference to FIGS. 4-6, the laser power output for various pumping configurations is seen. The pair of graphs in FIG. 4 correspond to the examples B and E in FIG. 6, i.e. to the side pumped configuration of the prior art. The pair of graphs in FIG. 5 correspond to the examples C and F in FIG. 6, i.e. the combination end and side pumped configuration of the present invention. Examples A and D in FIG. 6 correspond to the end pumped configuration of the prior art. The power output values in examples D, E and F are for a Nd:YAG rod active medium about 10 mm long in the preferred resonant optical cavity noted above and with the above noted preferred laser diode arrays 12 and 14 in FIG. 1. Examples A, B, C are for the same medium and laser diode arrays except that the resonant optical cavity has been modified to expand the radius of the TEM.sub.00 waist to 318 .mu.m.

In examples A and D, an end pump source, indicated as pump #1, has a continuous wave power output of 1 Watt, while a side pump source, indicated as pump #2, is either nonexistent or turned off. As is typical of end pumped laser configurations, the laser output powers are 100 mW cw and 277 mW cw for TEM.sub.00 waist spot sizes of 318 .mu.m and 130 .mu.m respectively. This low power output is in the TEM.sub.00 mode.

In examples B and E, the end pump source is either nonexistent or turned off, while the side pump source, i.e. pump #2, has a pulsed peak power output of 34 W and 37.8 W respectively for a pulse width of 160 .mu.sec. As is typical of side pumped laser configurations, the laser output powers are substantially higher than power outputs for end pumped lasers, having 3 W and 8.1 W pulsed power, respectively. However, as seen in FIG. 4, this side pumped laser configuration also has several disadvantages. In the upper graph of FIG. 4, side pump power output is zero watts prior to a time T.sub.0, rises to a finite positive value P.sub.0 at time T.sub.0, and remains substantially constant until time T.sub.3 when it again drops to zero watts. The pulse width, i.e. the duration of the pulse 38, is the time T.sub.3 -T.sub.0, typically about 160 .mu.sec as noted above. In the corresponding lower graph of FIG. 4, the laser power output is also zero watts prior to time T.sub.0 begin to rise when pulse 38 begins at time T.sub.0. Instead, there is a delay T.sub.1 -T.sub.0 before laser output begins to rise at time T.sub.1 followed by an interval from time T.sub.1 to time T.sub.2 in which the laser output exhibits large unstable spikes 40 due to gain switching. The laser output pulse 42 does not reach a stable power level P.sub.1 until time T.sub.2, then drops to zero at the end of the pump pulse 38 at time T.sub.3. Typically, the delay T.sub.1 -T.sub.0 is about 30 .mu. sec for 38 watt side pumping. In example B, where the TEM.sub.00 mode volume is enlarged, the laser output eventually reaches a stable TEM.sub.00 output. However, in example E, the laser output is not in the TEM.sub.00 mode.

In examples C and F, both the end and side pump sources, i.e. pumps #1 and #2, are active, pump #1 having an output of 1 Watt cw and pump #2 having a peak pulsed output of 34 W and 37.8 W respectively. The resulting laser power outputs in examples C and F are slightly higher than the outputs in examples B and E respectively. The power outputs are also slightly higher than the combined outputs of end pumped only and side pumped only lasers, i.e. the output in example C is slightly higher than the sums of the outputs examples A and B. This is due to the amplification process which already exists in the lasing medium that makes it unnecessary for the side pumping to reach threshold of the Nd:YAG medium. Further, the total laser output energy per pulse is greater by 80% in C relative to E because the delay between the start of the side pump's pulse and the start of the laser output pulse is substantially reduced, and typically negligible. This and other advantages are illustrated in FIG. 5. The side pump pulse 38 in the upper graph of FIG. 5 is the same as that in FIG. 4. In the lower graph of FIG. 5, the laser output power is at a finite positive value P.sub.2 prior to time T.sub.0 corresponding to the end pumped lasers in examples A and D. At time T.sub.0 or at a negligible time interval thereafter, the power exhibits a rise with a few minor spikes 46. The pulse quickly reaches a steady state 48 with a power level P.sub.3 that is slightly higher than power P.sub.1 in FIG. 4. The pulse remains substantially constant until time T.sub.3 when it drops back to the power level P.sub.2 exhibited prior to time T.sub.0. Thus, the power output of lasers of the present invention have greater pulse widths and smooth pulses that quickly follow the side pump pulses. Additionally, while in example E the laser output may never reach the TEM.sub.00 mode, the high power laser outputs in example C is in the TEM.sub.00 mode, and in F is mainly in TEM.sub.00 mode.

Although the side pumping laser diode array 14 in FIG. 1 may be operated with high peak power pulses, it can also be operated at a power level below the pump threshold of active medium 10. Since the end pump laser diode array 12 establishes continuous wave laser operation, any additional pump radiation from side laser array 14 gives rise to amplification in the lasing mode. So the side pumping laser diode array 14 can be modulated at high speed just above its threshold to create amplified pulses from the laser in FIG. 1 without any jitter. Any change in the amplitude of the pumping pulses results in a corresponding change in the intensity of the amplified laser pulses. This technique is useful for optical communication and other applications that require stable pulses with a TEM.sub.00 cavity mode. Typically, amplified modulation rates of up to 1 MHz can be achieved, with higher rates possible by optimizing the laser active medium, resonant optical cavity and the like to minimize delay and spiking. The frequency of the relaxation oscillation f.sub.ro is given by: ##EQU1## and .tau..perspectiveto.162 .mu.sec for Nd:YAG. For P/P.sub.th .perspectiveto.9, f.sub.ro .perspectiveto.0.46 MHz. With the above described configuration, the laser can be modulated at this frequency regime. Since the laser output follows that of the modulated pump source up to the relaxation oscillation frequency of the active medium, many types of modulation encoding are possible including amplitude modulation, pulse position modulation and pulse width modulation.

Alternatively, instead of having an end pumping laser diode array with a steady continuous wave output, the end pumping laser diode array can be biased above or slightly below the continuous wave pumping threshold and the injection current can be modulated with the appropriate information. As a result, the laser optically pumped by the laser diode arrays will emit a modulated light superimposed on the continuous wave TEM.sub.00 light. In laboratory experiments we have modulated the Nd:YAG medium up to the f.sub.ro for different P/P.sub.th ratios. We easily achieve 0.2 MHz repetition rate for the above described system.

While the laser of the present invention has been described in terms of the ebbodiment in FIG. 1, the combination side and end laser pumping configuration can also be used together with Q-switch or cavity dumping, longitudinal mode locking and frequency multiplying to achieve the desired laser performance. The pumping scheme of the present invention results in a laser with a stable desired transverse cavity mode, higher efficiency and power output, little or no delay, suppressed jitter or spiking, and a high frequency modulation capability.