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Final Report for Contract 

ototvne Laser -Diode - PumDed 

Solid State La ser Transmitters" 

Contract NAS7-999 

For period April 1987 - June 1989 

ft 

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Thomas J. Kane 

Emily A. P. Cheng 

Richard W. Wallace 

Lightwave Electronics Corporation 

1161 San Antonio Road 

Mountain View, California 94043 

(41 5) 962-0755 

June, 1989 

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Final Report for Contract 

Solid State Laser Transmitters" 

Contract NAS7-999 

For period April 1987 - June 1989 

ototvDe Laser Diode PumDed I .) 

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_- - 

Thomas J. Kane 

Emily A. P. Cheng 

Richard W. Wallace 

Lightwave Electronics Corporation 

1161 San Antonio Road 

Mountain View, California 94043 

(415) 962-0755 

DSTUCT: Monolithic, diode-pumped Nd:YAG ring lasers can provide 

diffraction-limited, single-frequency, narrow-linewidth, tunable output which is adequate 

for use as a local oscillator in a coherent communication system. We built a laser which had 

a linewidth of about 2 kHz, a power of 5 milliwatts, and which was tunable over a range of 

30 MHz in a few microseconds. We phase-locked this laser to a second, similar laser. This 

demonstrates that the powerful technique of heterodyne detection is possible with a diodepumped laser used as the local oscillator. 

Laser diode pumping of monolithic Nd:YAG rings can lead to output powers of 

hunhds of milliwatts from a single laser. We built a laser with a single-mode output of 

310 mW. Several lasers can be chained together to sum their power, while maintaining 

diffraction-limited, single frequency operation. We demonstrated this technique with two 

lasers, with a total output of 340 mW, and expect that it is practical for up to about ten 

lasers. Thus with lasers of 310 mW, output of up to 3 Watts is possible. The chaining 

technique, if properly engineered, results in redundancy. 

The technique of resonant external modulation and doubling is designed to efficiently 

convert the cw, infrared output of our lasers into low duty-cycle pulsed green output. This 

technique was verified through both computer modeling and experimentation. Further work 

would be necessary to develop a deliverable system using this technique. 

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I . Project Summary 1 

Table 1: Laser Performance 2 

IL Introduction 3 

1 . Relevant NASA Goals ............................................................... 3 

a) Diode-pumped solid-state lasers ............................................. 4 

c) Coherent communication ...................................................... 5 

d) Pulse Position Modulation .................................................... 6 

3 . Objectives Summary ................................................................. 6 

a) Completion of Local oscillator ............................................... 6 

b) Development of Modular High Power Single-Made Transmitter ........ 7 

c) Resonant External ModulationDoubling .................................... 7 

2 . Technical Background ............................................................... 4 

b) Diode-pumped single-frequency ring lasers ................................ 5 .. 

III . Local Oscillator 7 

1 . Improved MISER resonator design ................................................ 7 

2 . Jigging ................................................................................. 8 

3 . Feedback Measurements and Results .............................................. 11 

5 . Tests for Single Mode ............................................................... 16 

6 . Amplitude Noise Reduction ......................................................... 19 

8 . Laser Delivered to NASA ........................................................... 25 

.. 4 . Frequency Stability ................................................................... 13 

7 . Fast Tuning and Phase Locking .................................................... 22 

IV . High Power Module 26 

1 . Investigated Pumping and Resonator Geometries ............................... -26 

2 . Monolithic Rings with curved Surface ............................................ 28 

3 . All flat MISER ........................................................................ 30 

4 . Efficiency of flat resonator .......................................................... 32 

6 . Modular laser delivered to NASA .................................................. 41 

5 . Injection Locking and Chaining .................................................... 35 

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V . REMD (Resonant External Modulation and Doubling) 46 

1 . Mechanical Analogy .................................................................. 46 
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2. Possible Configurations .............................................. ........... .... 46 

3. Experimental Set-up .................................................................. 49 

4. Computer model. .. . . . . . . .. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .51 

5. Verification of program results with bench-top results . . . . .. . . . . . . . . . . . . . . . . . . . .54 

VI. Relation to Future Work 58 

1. Increasing power output.. . . . . . . . . . . . . . . . . . . . . . . .. . . .. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .58 

2. Modulation of output ................................................................. 58 

3. Untended operation .. . . .. . . .. . .. . . .. . . .. . ... ... . . .. .. .. .... ... . ... . .. .... . . .. .. .. . .. .59 

4. Diade lifetime and reliability .. ... . . ..... . . .. . . .. . ..... .. . . .. . . .. . . ... . .. . .. . . .. . . .. .59 

VII. Conclusions 59 

VIII. References 60 

Appendix 1 Fast frequency tuning and phase locking of diode-pumped Nd:YAG ring 

lasers 61 

Appendix 2 Resonant External Modulation and Doubling Model 64 

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I. Project Summary 

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The National Aeronautics and Space Administration (NASA) has the goal of developing freespace point-to-point communication systems using optical radiation. The advantage of free-space 

communication using optical radiation is due to the small antenna aperture needed for low 

divergence transmitted beams. Laser transmitters used for optical communication must have power 

of the order of 1 Watt . The beam must be diffraction-limited to be useful. Three modulation 

formats which have been proposed are phase modulation, frequency modulation, both known as 

coherent communication, and pulse position modulation, or PPM. This report describes progress 

in building lasers useful for &-space communication using these three modulation foxmats. 

Efficient, diffraction-limited all-solid-state lasers with power approaching 1 Watt at the near 

infrared wavelength of 1.06 microns can be built using the technique of laserdiode-pumping. The 

reliability of these lasers is essentially equal to that of the laser diode pumps used. Under this 

contract, we have developed diode-pumped lasers that have capabilities useful for both coherent 

communication and PPM communication through space. 

Monolithic Nd:YAG ring lasers can provide diffraction-limited, single-frequency, narrowlinewidth, tunable output which is adequate for use as a local oscillator in a coherent 

communication system. We built a laser which had a linewidth of about 2 kHz, a power of 5 

milliwatts, and which was tunable over a range of 30 MHz in a few microseconds. We phaselocked this laser to a second, similar laser. This demonstrates that the powerful technique of 

heterodyne detection is possible with a diode-pumped laser used as the local oscillator. 

Laser diode pumping of monolithic NdYAG rings can lead to output powers of hundreds of 

milliwatts from a single laser. We built a laser with a single-mode output of 310 mW. Several 

lasers can be chained together to coherently sum their power. We demonstrated this technique with 

two lasers, with a total output of 340 mW, and expect that it is practical for up to about ten lasers. 

Thus with lasers of 310 mW, output of up to 3 Watts is possible. The chaining technique, if 

properly engineered, results in redundancy. 

The technique of resonant external modulation and doubling is designed to efficiently convert 

the cw, infrared output of our lasers into low duty-cycle pulsed green output. This technique was 

verified through both computer modeling and experimentation. Further work would be necessary 

to develop a deliverable system using this technique. 

The research done under this Small Business Innovation Research (SBIR) contract has led to a 

successful line of commercial products sold by Lightwave. Almost 400 lasers have been delivered 

to customers in a variety of fields. A significant fraction of the deliveries have been to Japan and 

Europe. The "Phase III" self-sustaining commercialization program envisioned by the designers of 

the SBIR program is already well underway. 

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JX 1: 

A Local 0x2 'llator Best Resu It Delivered Un' 1t 4 

Wavelength 1.06 pn 1.06 pm 

output power 11 mW 4.5 mW 

Oscillating Mode "E&, single frequency 

Input pump optical power 30 mW 30 mW 

OpticalEfficienCy 37% 15% 

Input electrical power 162 mW 162 mW 

Electxical Efficiency 6.7% 2.8% 

Amplitude Stability (5Hz to 1 MHz, RMS) 0.03% 0.12% 

Frequency Stability 1 mSec 500 Hz 500Hz 

11 11 1 sec 20 kHz 20 kHz 

1 hour 10 MHz 10 MHz 

Thennal tuningrange 35 GHz 15 GHz 

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Thermal tuningresponsc time 0.2 sec 3sec 

Piem tuning range 200 MHZ 30MHz 

Piem tuning response time 3 cL= 3 cL= 

h-Power Module t 

Wavelength 1.06 pn 1.06 pm 

output power 311 mW 135 mW 

OsCillatingMode "E&, single frequency 

Input pump optical power 527mW 325 mW 

optical Efficiency 59% 41.5% 

Input electrical power 1.8 w 1.32 W 

Electrical Efficiency 17.3% 10.2% 

Amplitude Stability (5Hz to 1 MHz, RMS) 0.94% 3.4% 

Combined Output, 2 modules 340 mW* 260 mW 

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*340 mW is the combined power of the delivered units when pump diodes are run at a level 

which will reduce lifetime. 260 mW is the output from the same units at rated power. 

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II. Introduction 

1. Relevant NASA Goals 

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This contract work had as its goal the development of lasers useful as transmitters for freespace optid point-tepoint communication. 

NASA has many potential applications for an optical fke-space communications link. The 

specific goal of JPL is for fairly low bit-rate links (several MHZ) over extremely large distances, 

such as those contemplated for proposed deep space exploration probes. Other branches of NASA 

are interested in links between geo-stationary earth-orbiting satellites. These links require bit rates 

approaching 1 GHz. Optical links are very attractive because low divergence beams are possible 

with optical antennas of moderate size. The higher power possible on target with these low 

divergence beams more than compensates for the increased quantum noise present at optical 

frequencies. 

Four key requirements for a communication laser are beam quality, with a diffraction-limited 

beam preferred, high power, with a level near 1 Watt being desirable; good power efficiency, 

with 10% a good goal, and high reliability. Diode-pumped solid state lasers are promising on all 

four counts. Diode-pumped lasers, when pumped by wide-stripe laser diodes which are many 

times worse than the diffraction limit, still have output which is diffraction-limited. During this 

contract, we have built a diode-pumped laser which is diffraction-limited, which has an output 

power of 31 1 mW, and which has an output power equal to 17.3% of the input electrical power. 

These values are in the range required for space communications links. 

The output of the laser must be modulated in some manner to be useful for communications. 

Diode-pumped lasers cannot be modulated directly by simply modulating diode input current, 

because the dynamics of these lasers are quite slow, with time constants of a few microseconds. 

Other modulation schemes are required. Two possible modulation formats are phase modulation, 

where the infixmation is encoded on the optical phase of the transmitter laser, and pulse-positionmodulation, where several bits of information are sent in a single pulse, with the information 

encoded according to the timing of the pulse. The lasers we have developed are applicable to both 

these types of modulation. 

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2. Technical Background 

a) Diode-pumped solid-state lasers. 

Semiconductor laser diodes are now commercially available at very high power levels. The 

latest high-power laser from Spectra-Diode Laboratories, the largest U.S. manufacturer, operates 

at an output power of 10 Watts, continuous wave. This compares with a maximum available 

power of 200 mW only five years ago. These lasers are very efficient compared to other lasers, 

with an output power of 1 Watt possible with an input currcnt of 1.6 Amperes at 1.7 Volts. This 

is an efficiency of 37%. The reliability of semiconductor lasers can be good. Lifetimes of greater 

than 100,OOO hours are possible when diodes are operated at conservative power levels. These 

levels of efficiency and reliability are far better than is possible with any other kind of laser. 

Two significant disadvantages of high-power laser diodes are the lack of spatial and temporal 

coherence. Low spatial coherence is equivalent to a beam divergence which is greater than the 

diffraction limit. The most power which current laser diodes can achieve in a single diffractionlimited spot is about 100 mW. More powerful laser diodes have correspondingly larger beam size. 

Low temporal coherence, or wide linewidth, are characteristics of a laser which make it impossible 

to use phase modulation, or other coherent communication techniques. 

The output from laser diodes is can be efficiently matched to the absorption lines of Nd3+ in 

Yttrium Aluminum Garnet (YAG), and used to build efficient optically-pumped NdYAG lasers. A 

very significant advantage of diode-pumping is that it is possible to build a diffraction-limited 

diode-pumped laser using one or more laser diodes which are not themselves diffraction limited. 

As long as the diode light from the pumps is mostly absorbed inside the volume of the lowest 

order spatial mode of the pumped laser, the pumped laser will lase in a diffraction-limited mode. 

Since conversion efficiency from diode light to diode-pumped output power is typically in the 

range from 3096 to 5096, powerful and efficient diffraction-limited lasers can be built. 

One factor which is often ignod when the efficiency of diode-pumped lasers is discussed is 

the power used for temperature control of the laser diode. The laser diode must be temperaturecontrolled to within a l0C range to keep the output of the laser diode at the peak absorption of the 

solid-state laser material. If the correct temperature of the laser diode is far from the temperature of 

available heat sinks, the temperamantrol power may be significant. If the heat sink temperature 

is known in advance and does not fluctuate, then lasers can be specially selected at the desired 

wavelength, and the power needed for temperature control kept small. This may be the case for 

satellite-based systems. In field systems, where ambient temperature is variable, the diode 

temperature controller may be the largest user of power in the laser system. 

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b) Diode-pumped single-frequency ring lasers 

The local oscillator studied in Phase I of this SBIR contract was based on the MISER, a 

monolithic, isolated, single-mode, end-pumped ring laser. It was invented at Stanford University 

by Kane and Byer [ 11 during a successful effort to demonstrate a coherent laser radar system 

utilizing Nd:YAG lasers operating at 1.06 micron. The MISER ring laser, for which Lightwave 

has a license from Stanford, was invented to overcome two unavoidable problems with linear 

diode-pumped solid state lasers: multimode oscillation and sensitivity to optical feedback. 

Unidirectional-ring solid-state lasers were known to overcome these problems, but had the 

disadvantage of being complex devices with numerous discrete optical components. The MISER 

invention replaced all of these components with a single four-faceted crystal. This is a single 

crystal of NdYAG (neodymium doped yttrium aluminum garnet) cut and polished to form a 

complete resonator. Three of the reflections in the round-trip lasing path are total internal 

reflections, and the fourth reflection, the inputloutput surface, is dielectrically coated. 

Unidirectional oscillation is possible due to the nonplanar path. When placed in a magnetic field 

and pumped with a properly aligned diode laser, the MISER will oscillate unidirectionally in a 

single mode of very narrow linewidth, with a natural resistance to optical feedback. The MISER 

produced by Lightwave with Phase I support provided 2 mW of output when pumped with an 

inexpensive single-stripe diode laser. A major goal of the Phase 11 program was the further 

development of powerful single-frequency diode-pumped lasers. 

c) Coherent communication 

In a coherent communication link, the phase or frequency of the transmitted beam contains the 

information. This contrasts with an incoherent system, where the intensity (or the presence or 

absense) of the transmitted beam carries the information. This is similar to the distinction between 

F'M and AM radio transmission. Coherent communication has significant advantages when 

working against the noise of a bright background, or when more than one channel must be 

overlapped in space. Coherent communication requires that the transmitting laser be oscillating in a 

single frequency of narrow linewidth. At the detection end of the link a second, less powerful 

laser is used as a reference to detect phase or frequency changes in the transmitter laser. This laser, 

known as the local oscillator, must be a single fiequency laser of narrow linewidth. In addition, 

this local oscillator laser must be tunable, so that its frequency can track that of the transmitter. 

Under this contract we have made developments which have brought coherent diode-pumped 

space communication much closer to reality. 

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d) Pulse Position Modulation 

The technique of pulse position modulation (PPM) requires less average power than a binary 

code to transmit information. This is done at the cost of requiring more peak power. A PPM 

system operates at a low duty cycle. The time between pulses canies the information. If the pulse 

is short and the anival time is measured precisely, then many bits of information are carried on a 

single pulse. Against a dark background, a PPM system requires received powers which may be 

lower than one photon per bit of information. These systems are attractive for deep space probes. 

The problem with PPM is that it is only advantageous if it makes use of power efficiently. If 

power is wasted during the "off" part of the cycle, then a simple binary code may be better. Since 

laser diodes can not be run at peak powers significantly above their cw limit, there is a need for a 

technique for converting the cw or highduty-cycle output of diodes into the low duty cycle needed 

for PPM. We investigated a technique for converting the output of our powerful single-frequency 

lasers into randomly pulsed, frequency-doubled output appropriate for PPM. We call this 

technique Resonant External Modulation and Doubling. Our experiments and modelling convinced 

us that our technique is valid, but much more work is required to build a practical system based on 

this approach. 

3. Objectives Summary 

a) Completion of Local Oscillator 

The fmt objective was the completion of the low power local oscillator begun in Phase I. 

Although the device developed in Phase I was acceptable for some applications, there were several 

problems that needed more attention. Its sensitivity to feedback, the lack of fast tunability, and the 

si@icant amplitude noise due to relaxation oscillations were all areas that could be improved. 

The soldering and assembly techniques studied in Phase I needed improving as well. 

To accomplish these goals, the following tasks were to be performed. 

a. Develop a theoretical understanding and computer model of the monolithic nonplanar ring. 

b. Design miniaturized electronics for stabilization of MISER. 

c. Design and fabricate MISER resonator optimized for beam quality and power. 

d. Redesign of mechanical assembly and alignment hardware. 

e. Design electrical connectors and mechanical mounts for finished assembly. 

f. Test finished unit for power and stability. 

g. Deliver 1 unit to NASA. 

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b) Development of Modular High Power Single-Mode Transmitter 

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The second objective was the development of a laserdiode-pumped, solid-state laser module 

with high output power. Two or more of these modules combined would be suitable as a 

transmitter with powers nearing one Watt. The high power module should have the desirable 

characteristics of single-longitudinal mode, fundamental spatial mode, 1.06 micron output. The 

program to develop this module was as follows: 

a. Study and choose an optical beam combining geometry. 

b. Design pump module mechanics and optics. 

c. Design resonator structure. 

d. Design base plate and alignment jig. 

e. Acquire and assemble components. 

f. Test and evaluate system. 

g. Deliver to NASA. 

c) Resonant External ModulatiodDoubling 

The third objective was to thearetically investigate, computationally model, and experimentally 

venfy the technique of resonant external modulation and doubling. The intended steps to carry this 

out were as follows: 

a. Develop theory of coupled, modulated nonlinear resonators. 

b. Develop computer model based on theory. 

c. Experimentally verify theory and model. 

d. Test and characterize output of experimental system. 

e. C0mpat.e experimental results with computer model. 

III. Local Oscillator 

1. Improved MISER resonator design 

High resistance to optical feedback is a valuable trait for a laser to be used in a coherent 

communication system. This factor also improves the performance of the resonant doubling 

system, described later in this report. In cooperation with graduate students at Stanford, a 

mathematical model was developed which calculated the sensitivity to feedback of various designs 

of monolithic ring resonators. This work was recently published.[2] This model showed that the 

Phase I design was far from the theoretical optimum in regard to feedback resistance. Therefore, a 

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new design was developed and fabricated. New fabrication tooling had to be designed and 

constructed. 

Feedback resistance was not the only goal, however. Additional constraints were that the ring 

resonator fit into the previously designed package and that it be not too sensitive to small errors in 

fabrication and to em in the phase of the reflectivity of the output coupler. Also, the new 

resonator was designed to be tuned with a mechanical force, which meant that the top of the crystal 

had to be a non-optical surface. The resulting design, shown in Figure 1, had a resonator that was 

barely nonplanar compared to the old design. Table 2 gives the design parameters of all the 

nonplanar rings which we know of, including the two designs developed at Lightwave under 

NASA support. The definitions of the parameters are from Ref. 2. The half-size laser was 

developed under a different contract, but is included for completeness. 

TMLE 2: DIFFERE NT RING DESIG NS 

Slol eA - 6 Round-triD 

First Stanford Design 7.8" 900 36.5 mm 1.8 mm 84.3 mm 

Second Stanford Design 160 900 10.5 mm 1.5 mm 28.6 mm 

Hewlett-Packad Design 3CP 1.25" 4.23 mm 1.77 mm 15.8 mm 

Third Stanford Design 300 1.05O 4.0 mm 2.33 mm 15.8 mm 

Lightwave "Old-Style" 22.9" 900 3.5 mm 1.03 mm 11.2 mm 

Lightwave "New-Style" ZtOB 3.5 mm 1.41 mm 11.5 mm 

New-Style, Half-Scale 2204 1.75 mm 0.71 mm 5.8 mm 

2. Jigging 

New tooling had to be designed to fabricate ring resonators crystals of the new, feedback 

resistant design. The tooling parts were machined in the same manner as the previous tooling used 

for the highly non-planar design. The precision-ground block holds the smaller pieces at a 

preciselydefined angle. There are three distinct sites on the large block, one for each of the flat 

surfaces which must be ground onto the ring resonator. The laser crystals are bonded to the 

smaller pieces throughout the process, so the relative angles are those of the smaller pieces on the 

large block. This tooling has been quite successful. Yields for most types of crystal built have 

been good. The exceptions are described later in this report. 

We also built a system for permanently aligning lasers with the pump beam. During 

alignment, the crystals are held with a vacuum chuck and positioned with all six degrees of 

bedom. A heater pad under the crystal keeps a blob of solder melted while aligning the laser, and 

when the solder freezes the crystal remains in the desired alignment. This soldered alignment 

technique has been quite successful. A person skilled in alignment can get a crystal lasing in about 

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original highly non-planar design optimized design for maximum 

feedback resistance 

Figure 1 depicts both designs of the MISER crystal. The design on the left has a highly 

non-planar beam path. The design on the right was optimized to minimize feedback 

sensitivity. Some features common to both designs are: the inputloutput mirror (curved 

surface) has a highly reflective dielectric coating; the three other bounces are total 

internal reflections, and the input and output beams are 90° apart. 

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ORIGINAL PAGE IS 

OF POOR QUALITY 

Figure 2. Fluorescence path in a lasing crystal, viewed from the front curved surface of the 

resonator. This photograph was taken with an infraredsensitive video camera, used 

during laser alignment. The top vertex is a total intemal reflection from the top, polished 

surface of the crystal. The bright spot in the center of the lower side of the "triangle" is the 

input/output point on the front, coated surface. The additional lines on the right and left 

edges of the triangle are merely secondary reflections in the crystal when viewed from this 

angle. 

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five minutes. The use of infrared-sensitive video cameras to allow the aligner to see the 

fluorescence path in the crystal is a valuable technique, which allows skilled aligners to quickly 

make adjustments which lead to lasing. Fig. 2 is a photograph of the fluorescence in a crystal 

which is being aligned. 

3. Feedback Measurements and Results 

Feedback degrades the performance of all types of lasers. In a linear laser, even a very small 

amount of feedback will pull or split the frequency of the laser, reducing its coherence. These 

effects are not observed in our ring resonators. In a unidirectional ring laser, the feedback injects 

power into the ring resonator which travels in the opposite direction from the laser. True 

oscillation in the wrong way is not possible, since the threshold for the right way is slightly lower, 

and it oscillates first and saturates the gain. Still, the wrong-way resonator has a very high Qfactor, so power injected from outside the laser into the wrong direction will build up. This power 

competes with the oscillating right-way power for the excited atoms. When the wrong-way power 

is of the same order as the oscillating right-way power, there must be a reduction in the right-way 

laser output. Thus we observe that at feedback levels above a certain amount, the laser output is 

reduced. Amplitude stability is also reduced, since the amount of feedback will be fluctuating. 

We measured the effects of feedback on Nd:YAG monolithic ring resonators of both types. 

The set-up for measuring feedback is shown in Figure 3. This setup takes advantage of the fact 

that only light spatially matched to the laser resonator will build up in the resonator. Light reflected 

from a mirror at a beam waist will be spatially overlapped nearly perfectly with the incoming beam. 

As the distance from the beam waist to the mirror is increased the overlap of the two beams will be 

reduced in a well-defined way. The exact amount of feedback is difficult to know exactly. The 

virtue of this setup is that feedback is precisely controlled by mirror position in a way that is the 

same for both lasers. 

The focussing lens was AR coated for 1.06 pm A 99% reflecting mirror at normal incidence 

was moved along the axis of the beam, changing the amount of light spatially matched back into 

the laser. Feedback affected the highly nonplanar and the slightly nonplanar lasers differently as a 

function of mirror position. The old highly-nonplanar design showed effects at a feedback level of 

0.1 % The newer barely-nonplanar design was substantially more resistant to feedback. 

Nevertheless, significant effects are seen at a feedback level of 1%. We will experiment with 

materials other than NdYAG which may yield more feedback-resistant designs. 

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99 % reflector 

laser 

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lens translation 

Figure 3 shows the experimental set-up for feedback measurements. The beam from the 

laser is focused onto a 99% reflecting mirror. The amount of feedback varies with the 

position of the mirror, with the greatest amount of spatially-matched feedback occurring 

when the mirror is at the focus. The feedback drops off in a calculable way as the mirror is 

translated. 

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4. Frequency Stability 

Many times in the development of the ring lasers, experiments such as those shown in Fig 4 

were set up to measure the frequency stability of the lam. The experiment shown is a beat note, 

or heterodyne, experiment, where the beams of two lasers are combined and the frequency 

difference between the two lasers is observed in order to measure the frequency stability of the 

frequency Merence between the two lasers. The beams from the two lasers are collimated with 

50 mm focal length lenses and crossed at a 50-50 beamsplitter. The transmitted beam from one 

laser and the reflected beam from the other are carefully overlapped. This can be done with either 

of the two sets of beams that leaves the beamsplitter. The aligned beam is then detected on a 

photodiode, the output of which is connected to an oscilloscope or a fast frequency counter. When 

the frequency of one laser is tuned close enough to the other, the beat frequency of the two lasers 

can be observed. 

Initially, it was suspected that the long term frequency drift was due to air currents in the laser 

head package. The laser diode is run considerably below mom temperature, while the MISER 

crystal is above room temperature, and so thermal gradients are expected to cause convection 

currents. To test this theory, a roughing pump capable of pulling a 20 mTorr vacuum was used to 

evacuate the hermetic laser assembly enclosure. A sealed, evacuated unit and a standard, nitrogen 

purged unit were set up in a heterodyne configuration described in the previous section. The long 

term frequency stability was measured and compmd to that of two standard units. There was no 

noticeable improvement due to the evacuation of the laser enclosure. Other sources of frequency 

drift predominate. 

We found that a major source of long term frequency drift for the Phase I design was aging of 

the temperature sensor. We were using an unencapsulated thick film thermistor, which we found 

to be quite noisy, and to exhibit substantial long term drift. We replaced these devices with 

platinum thermistors. These commercially-available devices use platinum sputtered onto a ceramic 

substrate as a temperature sensor. The platinum is inert, so it has excellent long-term stability. 

Long-term frequency stability was improved from 50 MHz/hour drift to less than 10 MHz/hour by 

this replacement. 

We reported on the frequency stability of the laser we built in a paper published in Optics 

Letters in April 1988. This paper is included as Appendix 1. We used a measure of frequency 

stability known as the Allan Variance. The Allan variance gives expected frequency change as a 

function of delay time. To measure the Allan Variance, the frequency of an oscillator is measured 

once with a frequency counter, and then again after the delay time. The expected square of the 

frequency change is the Allan Variance of the oscillator for the specified delay time. Plotted in Fig. 

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photodiode 

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laser 

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Figure qshows the experimental set-up for measurement of frequency stability. The beams 

from two lasers are combined at a 50-50 beamsplitter and detected on a silicon photodiode. 

When the beams are spatially overlapped correctly, modulation depth of the detected beat 

frequency is in excess of 85%. The signal was analyzed by a frequency counter and 

accumulated on a personal computer. 

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Fig. 5. Root Allan Variance is a measure of how much laser frequency is 

expected to change (RMS) in a given time. For times between 1 psec and 1 

msec the frequency change is below 1 kHz. 

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medium, and long term frequency stability are defined by the frequency variation of the beat 

frequency over lmsec, 1 sec, and one hour. These values, plotted in Figure 5, are given in Table 

3. 

TABLE 3: FlREouENcy ST ABILITY0 F LOCAL OSCILLATOR LAS ERS 

1 msec 500 Hz 

1SeC 20 lrHz 

1 hour 10 MHZ 

5. Tests for Single Mode 

One of the most important requirements of coherent communication is that the laser oscillators 

used oscillate in a single mode. We used sets of fixed-length solid etalons to determine the number 

of laser modes oscillating. The arrangement shown in Figure 6 is used. The laser beam passes 

through a diffusing element, such as teflon tape or frosted glass, and then through a solid FabryPerot etalon. An etalon is an interferometer composed of two flat and parallel partially mirrored 

surfaces, on a piece of optical glass [3]. Light of a single fiequency passing through an etalon 

creates a set of bright and dark fringes as the angle from the axis changes. This gives rise to a 

pattem of circular rings as seen in Figw 7. If the laser is not single mode, this pattern changes to 

grouped sets of rings, instead of the sharp, single concentric circles. With this method of mode 

detection, another mode at 1% of the power of the main oscillating mode is detectable. 

We found that the vast majority (>95%) of ring lasers produced from Nd:YAG at the 1.06 

micron wavelength lase at a single frequency without difficulty. There are problems at the 1.32 

micron wavelength of NdYAG, but that is beyond the scope of this report When multiple lasing 

frequencies are observed, it is due to either the presence of multiple axial modes, or to the 

presence of multiple transverse modes. 

The usual cause of multiple axial modes is that the laser is near a mode-hop. Figure 8 shows 

the wavelength of a typical ring laser as a function of temperature. The linear scan occurs in 

regions where a single mode is oscillating, and the frequency of that mode is being changed by 

thermally expanding the monolithic resonator cavity. As the laser gets warmer, the resonator 

expands, and the wavelength of the resonator mode expands accordingly. This increase in 

wavelength with temperature corresponds to a decrease in laser frequency. The coefficient is -3.1 

GHgC for Nd:YAG at 1.06 microns. Eventually the mode scans to a point where it is off the 

peak of the NdYAG gain, and another mode on the other side of the gain peak has almost the same 

gain. Over a narrow range of temperatures, both will lase. The single-mode region is about 5 OC 

wide, so it is quite easy to regulate the temperature for single mode operation. The overall slope of 

16 
ORJGINAC PAGE 

BLACK AND WHITE PHOTOGRAPH 

diffusing 

* element 

etalon 

e 

Figure 6. Experimental set-up for determining the mode quality of a laser. 

e 

Figure 7. Fabry-Perot interferograms. When a laser is running in a single mode, the sharp 

concentric rings as shown on the left appear. The fringe pattern on the right indicates laser 

oscillation in two adjacent des. 

e 

17 
20 

15 

n 

10 0 

w 

05 

2 

a 

I 

00 * 

0 

3 

0 

W 

pE -10 

IL 

U 

E -5 

-1 5 

-20 

30 35 40 45 50 55 

CRYSTAL TEMPERATURE ( C) 

Fig. 8. Temperature tuning of monolithic Nd:YAG resonator. In straight 

segments laser frequency tunes at -3.1 GHz/C. Discontinuities are mode-hops. 

In dashed regions near mode-hops two frequencies are oscillating. 

18 
0 

0 

e 

the Nd:YAG gain curye causes each successive mode to scan over a lower range of frequencies as 

temperature is raised. 

The usual cause of multiple transverse modes is that the pump beam is too big, or that it is not 

aligned properly with the monolithic resonator. Either problem will lead to significant energy 

storage and gain outside of the 'EM.,,,, mode volume of the laser resonator. Higher order spatial 

modes will oscillate. At high levels of power this will cause the beam to look bigger. Proper 

alignment, and proper choice of laser resonator p&ters, will eliminate higher order transverse 

modes. Later in this report we will discuss the techniques we used to maximize the size of the 

'EMoo mode in our ring resonators, and thus to achieve TE& output with high power laser 

diodes and their correspondingly large pump beams. 

6. Amplitude Noise Reduction 

Amplitude fluctuations in solid state lasers are predominantly due to a mechanism known as 

relaxation oscillations. These oscillations are due to a resonance between energy stored in the laser 

cavity as photons and energy stored in the laser medium as excited atoms. The frequency of 

relaxation oscillations for lasers of our design is between 200 kHz and 400 kHz. For a given laser 

at a given level of power the frequency is constant, and the bandwidth of the fluctuations is about 

10 kHz. At frequencies above the relaxation oscillation frequency the amplitude fluctuations drop 

off quickly, so that the laser output is quantum-noise-limited at frequencies above 40 MHz. The 

total amplitude fluctuation in a band from 5 Hz to 1 MHz is 0.2% RMS. This is improved from a 

typical level of 0.5% RMS in the lasers made in Phase I. This improvement was brought about by 

reducing the output coupling of the lam to a point slightly below the level which optimizes 

power. This brings the laser more times above threshold and thus reduces amplitude fluctuations. 

We decided that for ultimate control of noise an active feedback system would be necessary. 

A circuit was designed to monitor a portion of the 1.06 pm output beam and adjust the laser diode 

to correct for fluctuations. Any increase in 1.06 pn laser output was countered by decreasing the 

diode current. Another method was investigated in which the diode light was monitored and used 

to generate a correction signal to the diode current. However, this method was discarded because 

the fluctuations in diode power were very small, even though this caused most of the amplitude 

noise in the ring laser. The 300 lcHz relaxation oscillations are easy to detect and provide a larger 

signal with which to correct the diode power. Therefore, the best design incorporated the large 

gain which the relaxation oscillations provided to this negative feedback loop. 

To test this method, a small amount of the laser output is diverted with a beamsplitter to a 

photodiode, which is connected to a high-gain, low-noise amplifier, as shown in Figure 9. The rest 

of the laser output was monitored on an independent photodiode. The amplifier was AC coupled 

to the laser diode, while the DC current to the diode came from the standard power supply. With 

19 

e 
ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAP# 

Figure 9. The local oscillator with quieting electronics in schematic and photographic fann. The did pump beam is focused into the MISER crystal with a Selfoc (gradedindex) lens. Most of the 1.06 p.m output beam passes through the beamsplitter, but a small 

amount is diverted to the photodiode. The electronics then provide a correction to the laser 

diode current to reduce any amplitude fluctuations. 

20 
e 

e 

0 

the amplifier turned off, the 1.06 pm amplitude modulation was 0.3% RMS. With the amplifier 

turned on, the modulation was reduced to 0.05% RMS. This modulation was spread out over 

frequencies up to 1 MHz, instead of being concentrated at the relaxation oscillation frequency. 

The circuit used in these initial tests was then hybridized by Hybrid House, a San Jose 

manufacturer of custom hybrid circuits, on a 1" by 0.5" alumina substrate and placed in the laser 

head. Miniature optics, such as the beamsplitter and focusing lens, were used. The standard 

package which houses the laser head assembly was modified to pmvide room for the hybrid circuit 

and associated optics. The first laser built using the hybrid electronics had RMS noise in a 5 Hz to 

1 MHz bandwidth of 0.28% with the hybrid circuit turned off, and 0.03% with the electronics 

turned on. Figure 10 shows the spectrum of amplitude fluctuations with and without the quieting 

circuit in operation. The unit packaged for shipment to JPL had a noise level of 0.12%. The 

finished unit was delivered to JPL in February 1989. 

-- 

e 

e 

e 

Figure 10. Spectnun of amplitude fluctuations with and without the noise quieting circuit 

in option. The relaxation oscillation frequency can be easily seen as a sharp peak in the 

unquietcd spectrum. 

GRlGlNAL PAGE 

BLACK AND WHITE PHOTOGRAPH 21 
7. Fast Tuning and Phase Locking 

The non-planar design of the new feedback resistant MISER resulted in a large non-optical 

surface on top of the crystal. This allowed a piezo-electric element made out of PZT-5H ceramic 

material to be bonded to the resonator, as shown in Figure 11. This piezoelectric tuner, a .01" 

thick, .20 square plate, was purchased from the Vernitron Corporation of Bedford Ohio. The 

PZI' material was poled in its thin dimension, and electrodes were plated on its large surfaces. 

When a voltage is applied mss a PZI' plate, its area changes. In this case, the PZI' is epoxied to 

the rigid NdYAG crystal, and cannot move. Instead, it applies a force to the NdYAG, which then 

undergoes a very small expansion or contraction. This causes a change in the optical path length of 

the laser cavity, so that the resonant frequency of the single oscillating mode changes as well. 

Lightwave received a patent on this laser tuning technique [4]. 

Figure 11. Local oscillator with a piezoelectric tuning element bonded directly to the top, 

non-optical surface of the resonator. When a voltage is applied to the tuner, the FZI' 

material expands or contracts in the flat direction, applying a force to the crystal, thus 

changing the optical path length of the resonator. 

CR1GINAL' PAGE 

22 BLACK AND WHITE PHOTOGRAPH 

0 

0 

a 

a 

a 

(I 
e 

* 

0 

With a standard unit and this m-tuned unit configured in a beat note experiment, the tuning 

rate per volt applied to the PZI' was measured at 1 MHdvolt. With a +15 Volt step function 

applied to the PZT the tuning range was 15 MHz, and the response time was less than 10 psec, as 

shown in Figure 12. The rate of modulation is limited by the mechanical resonance of the YAG 

crystal. Ringing occurs when voltages with Fourier frequency components higher than 400 lcHz 

are applied, which can be seen in Figure 12a. The maximum voltage that can be applied to the 

piezo-electric transducer is around 100 Volts, which gives a tuning range of +/-200 MHz. At 

larger voltages, the piem element starts to become depoled and the tuning rate per volt decreases. 

This fast, precise tuning made it possible to phase lock two lasers with a simple, electronic 

feedback loop shown in Figure 13. The difference frequency of two lasers, one piezo-electrically 

tuned, is detected on two independent photodiodes. The output of one of these is filtered with a 1 

WZ resistor and a .47 pf capacitor, and applied to the PZT element. The other photodiode is used 

to independently monitor the optical signal. The lasers stayed phase-locked together for periods of 

minutes with RMS phase noise of 29 milliradians. Our work with fast tuning and phase locking 

was reported in the Optics Letters paper [5] which is included as Appendix 1. 

Bonding the piem element to the bottom of the YAG crystal was tried, to allow this method of 

tuning to be applied to the original resonator design, which has an optical surface on top. The 

tuning rate decreased to 300 kHzholt. This was probably due to the expending of force on the 

copper base which was bonded to the other side of the transducer. 

0 

e 

901 I 

-5 0 5 10 15 20 25 30 35 40 

TIME, MICROSECONDS 

Fig. 12. Frequency as a function of time following a voltage step applied to the 

PZT-tunable laser. a) a 1-psec voltage rise leads to ringing of the crystal. b) 5- 

psec and c) 15-psec rise leads to clean tuning of frequency. 

23 
oscilloscope 

second 

photodiode 

stable laser 

IGmW 

7 I 

first 

beamsplitter 

-1 

piezo-hectric 

transducer 

Figure 13: Two lasers phase locked by feeding a filtered signal from a photodiode directly 

to the piezo-electric element that tunes one of the lasers. 

24 
e 

8. Laser Delivered to NASA 

e 

a 

The local oscillator laser delivered to NASA in February 1989 met the following 

Power 4.5 mW 

Frequency stability: 

specificationS: 

1 msec 500 Hz 

1SeC 20 kHz 

1 hour 10 MHz 

RMS Amplitude noise, SHZ-lMHz 0.12% 

Single-mode tuning range, thd 

PZT tuning range with +/- 15 Volts applied 

Polalizatioil 99% linear 

-Quality TE- elliptical, 1.25: 1 

This laser has essentially all the propexties required of a laser to be used as the local oscillator 

in a coherent communication system. The only property of this laser which may not be adequate is 

expected lifetime and probability of failure. Further work will be required before these lasers have 

the very high reliability required for communication in space. Diode manufacturers currently 

estimate 50,000 hours mean time between failures. However, a large MTBF is not all that is 

required - a very low probability of premam failure is also important. We do not currently use 

diodes which are space qualified. More reliable diodes would have to be used in any space 

system. We believe that all the components of our system other than the diodes are very unlikely 

to degrade. A possible exception to this is the glue used for purposes such as attaching the fasttuning piezoelectric element to the laser crystal. These aspects of the laser must be studied further 

before use in space becomes areality. 

15 GHz 

30 MHz 

25 
IV. High Power Module 

1. Investigated Pumping and Resonator Geometries 

The goal of this part of our research and development program was to find a way to combine 

the output of a number of laser diodes in a way that could reach an output power of 1 Watt at 1.06 

microns. We proposed to find a "modular" approach, meaning that additional increments of power 

could be added by combining "modules" in some configuration. In addition to high power, an 

additional goal of the modular approach was to provide redundancy. We initially examined some 

approaches which involved a number of laser diodes pumping a single piece of YAG. One way to 

do this is by polarizationcombining. Laser diodes are fairly well polarized, so it is possible to 

perfectly overlap two beams in space by placing one pump laser so that it passes through a 

polarizing beam splitter before reaching the YAG, while the other, oppositely polarized pump is 

reflected by the same beam splitter before reaching the YAG. It is also possible to pump a rod 

from both ends, so that when used with polarization-beam-combining up to four lasers can be 

incident on the same rod. It might seem that a four-faceted ring of the type we use could be 

pumped at all four facets, but this is not the case. Three of the reflections are total internal 

reflections, so no external beam is colinear with the internal beam. However, at the reflection point 

where pumping and output coupling occur, it is possible to pump from two directions, with one of 

them being along the path of the output beam, but directed oppositely from the output beam. A 

piece of optics which reflects at the pump wavelength and transmits at the lasing wavelength is 

required to keep the output beam from being directed back to the pump laser. 

A modification of our standard ring design was considered, which would have more surfaces 

reflected by dielectric coatings rather than by total internal reflection. Four pump beams could 

enter at each (non total-internal) reflection point. End-pumping is possible not only with rod and 

ring resonators but also with zig-zag slabs as well. In Figure 14, each reflection point of the 

zigzag slab can be pumped with four diodes. Each pair of diodes is polarization coupled, and each 

direction of the bounce is pumped. In theory, this would allow large amounts of pump power with 

which to excite a high power laser. 

Lightwave has had experience with geometrical and polarization pump beam combination 

techniques. These techniques work well, but they require a substantial amount of mechanical 

engineering in order to be successful. Our experience had been that by the time a product is 

engineered for use with two diodes of a particular power, a single diode was available with the 

same power. This new, more powerful diode was not compatible with the carefully engineered 

beam combining optics. Lightwave essentially threw away a completely engineered Q-switched 

26 
e 

polarizing 

0 

I, 

0 

diode 1.06 vm 

output 

e 

0 

laser 

combiner 

Figure 14 shows two possible configurations for end-pumping YAG with multiple 

laser diodes. A linear laser rod is pumped with four laser diodes using 

polarization combining. The zig-zag slab on the bottom is shown end-pumped 

with four laser diodes at one of the non-total internal reflection points, but could 

be pumped at the other vertices, in both directions, as well. 

a 

27 

e 
laser design using beam-combining optics. The design was obsolete by the day it was complete, 

due to the availability of 1-Watt diodes. That system used two 200-mW diodes. This experience 

led us to take the following approach: We would seek to build the biggest possible laser using a 

single diode pump before we started using beam-combining techniques. 

Due to the previous success of the MISER, the nonplanar monolithic ring resonator was 

studied for use at high powers. The first experiment was to pump one of the monolithic rings 

designed for use with a 30-mW laser diode with a larger pump laser diode. We obtained a laser 

diode from the Sony Corporation with an output of 175 mW. This laser diode had an emitting 

aperture of 50 microns, rather than the 2 or 3 micron aperture of a 30 mW "single-stripe" laser. 

The light from this extended aperture cannot be focused to a small spot as easily as the light from 

the single-stripe diode. A simple single-lens imaging system such as is used for our single-stripe 

laser will lead to a pump beam substantially larger than the fundamental mode of the laser 

resonator. If the pump beam is larger than the lasing mode, higher-order spatial modes will 

oscillate. We designed an imaging system which used a pair of spherical mirrors to reshape the 

beam in a way that minimized the spot size of the light from the 50-pm aperture of the Sony laser 

diode. The ellipticity of the pump beam was reduced by a factor of four, and the astigmatism 

eliminated. A patent on this imaging system has been applied for. Using this reshaped pump 

beam and a standard design ring laser, we were able to obtain an output power of 55 mW. 

However, the output was not in a single transverse mode. Additional transverse modes were 

clearly visible as additional rings when the laser beam was passed through an etalon. 

These early ring experiments did not lead to single mode power, but the laser was 

unidirectional and there were no multiple axial modes. We decided that the MISER approach 

would be workable if the TEb mode volume could be expanded to include all of the pumped 

volume. Our efforts were concentrated on designing monolithic rings with greater TE& mode 

volume. 

2. Monolithic Rings with Curved Surface 

The matching of the pump beam to the TEM, mode volume is one of the critical aspects of 

the design of diode-pumped lasers. If the pumped volume extends beyond the TE& mode 

volume, then the TE& mode cannot extract all the stored energy. Gain remains in the laser 

material, which pumps higher order transverse modes. The laser will not be difktion-limited or 

single frequency. If the TE& mode volume is too large, then there may not be adequate gain. 

When designing for low power pumps, the problem of low gain is important. With high powered 

pumps, the problem of multiple transverse modes is much more serious. The primary problem in 

the design of diffraction-limited high-powered diode-pumped lasers is the problem of minimizing 

(I 

4 

4 

4 

4 

28 
e 

a 

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0 

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e 

a 

0 

the pumped volume while maintaining a "EM, made volume big enough to avoid higher order 

modes. 

The beam reshaping geometry described in the previous section minimizes the cross-sectional 

area of the pump beam from 50 pm aperture laser diodes over the absorption length of 806 nm 

light in N&YAG (about 2 mm.) The pumped volume is thus minimized. The design problem is to 

maximize the cross-sectional area of the TEMoo mode. 

The square of the beam radius w for the mode of a monolithic ring laser, for the case where 

b>L, is 

where k is the wavelength of the laser in air, n is the index of refraction (1.82 in YAG), R is the 

radius of curvature of the curved surface of the resonator, and L is the round-trip path in the 

resonator. To increase w, it is necessary to increase either R or L. We have tried to increase R 

rather than L in order to maintain the small size and resultant large mode separation of our rings. 

All the rings built under this contract have path length L equal to 11 mm. The three values of R 

and the carresponding values of w art given in Table 4. 

w* = (Wnn)[(~)~n] (1) 

* 1 .. f li * ' 

i 

.. 

16 mm 42 pm 

6omm 59 pm 

160 mm 75 pm 

The mode radius incrtases very slowly as the radius of curvature of the resonator is increased, 

in fact, Eq. 1 shows that w increases as the fourth root of R The 16-mm curvature resonator was 

used with the 30 mW pump. This resonator did not run single mode with the 175-mW Sony laser. 

The 160-mm curvature resonator lased single mode at maximum power with that pump. The best 

output obtained with the Sony pump was 70 mW. Our commercial laser, the Model 120-03, 

makes use of Sony pumps and 160-mm radius-of-curvature resonators. The specified output 

power is 40 mW, which is obtained without difficulty. 

We have fabricated monolithic resonators of both the old (90' out-of-plane) and new (4' outof-plane) designs, at radii of both 16 mm and 160 mm. All of the 16-mm resonators lased 

successfully, and all of the old-style resonators lased successfully. Many of the 160-mm, newstyle resonators failed to lase. Those that did had beam paths which were asymmetrical, though 

the design path has perfect symmetry. The greater difficulty of fabricating longer radius resonators 

is not surprising. As the radius of curvature is increased, the shift in the lasing path due to a miscentration of the center of curvature was expected to grow pportionally. The change in position 

29 
Ap of the self-replicating ray in a two-mirror resonator, with one mirror flat and the other of radius 

of curvature R, is given by 

where AC is the error in angular alignment of one of the mirrors. Our resonators have a thickness 

of 2 mm, with the nominal lasing path exactly centered, 1 mm from each surface. Thus if the 

sphere center moves by 1 mm, the beam path moves out of the laser crystal, and the laser will not 

lase. For a radius of curvature of 16 mm, the angular centration error corresponding to a 1 mm 

motion is sin-l( 1/16), or 3.5', which is not difficult. For a 160 mm radius, we get 0.35'. This is a 

demanding tolerance. 

Thus a simple two-mirrOr resonator with both faces perfectly flat is nearly impossible to build, 

since unless fabrication is perfect, no ray will have a closed path. What was surprizing was the 

fact that old-style resonators of both radii showed essentially no evidence of fabrication error. We 

discovered that it had been theoretically predicted that highly out-of-plane ring resonators with an 

even numk of reflection points would exhibit high tolerance for fabrication error [6]. Equation 

(2) is approximately valid for planar resonators, but for nonplanar resonators the change in the 

position Ap of the resonator beam is smaller. According to [6], the tolerance for highly out-ofplane resonators is given by an equation similar in form to Eq (2), but with R replaced by L, the 

round trip path length. This means that for our design, with kl1.5 and the maximum value of 

Ap=l mm, we can have an angular error as large as 5', no matter how large R is. Thus in theory 

very large values of R can be used, with correspondingly larger values of beam radius w. 

Ap = R sin-l(AC) (2) 

3. All flat MISER 

In order to obtain higher output powers, larger pump diodes are needed. Unfortunately, 

higher power diodes usually have larger emitting apertures, and thus can not be focussed to spots 

as small as are possible with lower power diodes.. This means that the mode volume of the 

chosen resonator must also be larger if oscillation in the TE& mode is to be maintained. One 

means of increasing the mode volume in a cavity is to lengthen the resonator. We preferred to use 

a short resonator, because the laser remains small and the mode spacing and range of tunability 

correspondingly large. Another way to increase the mode volume is to increase the radius of 

curvature of the output coupler. The radius of curvature of Lightwave's commercial low power 

laser is 16 mm, while for the commercial high power (40 mw) laser the radius is 160 mm. Larger 

radii of curvature at first did not seem practical, for the reason described above, that is, because the 

tolerances on angular centration become extreme. Once we understood the tolerance-reducing 

effects of highly nonplanar ring resonators, we became interested in resonators with very large 

curvature. The limit of large curvature is, of course, a planar surface. According to Eq. (l), a 

resonator with planar surfaces will have an infinite value of the mode radius w, and the mode will 

30 
e 

e 

e 

e 

e 

a 

be unconfined and unstable. Any small effect which corresponds to a positive radius will stabilize 

the resonatr, any negative effect will destabilize the resonator, so that no laser is possible. 

The predominant effect, which turns out to have a positive, or stabilizing effect on the laser 

resonator, is the effect of temperature gradients on the laser material. When high-power laser 

diodes are focused inside the YAG crystal, thermal gradients are mated, with a higher temperature 

on the axis of the pump beam. A higher temperature leads to an increase in the index of refraction 

of the material, which gives rise to a positive lensing effect, equivalent to a positive radius of 

curvm. We have developed a simple model of this lensing effect. This thermal lensing effect is 

important because it limits the power available from a single ring resonator. If the lensing is too 

strong, the TE& mode radius y will shrink, to the point where some of the pumped volume is 

outside the mode. This will lead to higher-order transverse modes. 

The assumption which permits an approximate understanding of thermal focusing in diodepumped lasers is that the thermal focusing that determines mode volume is due to the effective lens 

which exists right on the axis of the mode. Focusing is due to a quadratic index variation, so it is 

assumed that the parabolic index of refraction gradient that fits at the center of the mode is the one 

that determines the focussing. Thus we ignore the complex temperature distribution which exists 

in the pumped crystal, and pay attention only to the second derivitive of the temperature field right 

at the center of the mode. This simplifies calculations greatly, and is probably not too far off, since 

most of the optical power is at the center of the mode. 

The thermal lensing depends on the heat deposited in the crystal per unit cross-sectional area. 

The distribution along the axis is not significant - only averages along the axis. If the pump laser 

has a power P, then the heat deposited per unit cross-sectional area, Q, is given by 

where wp is the radius of the pump beam and q~ is the fraction of the pump power which is 

converted to heat, typically 25%. According to Koechner [7], the focal length of a rod uniformly 

heated by a steady-state heat per unit area of Q is 

f = 2 k/ ( Q dn/dT ) 

where k is the thermal conductivity of the material (0.13 Watts/cm-K for YAG) and dn/dT is the 

coefficient of change in index of refraction of the material (7.3~10-6 K-1 for YAG.) A lens of 

focal length f has the same focussing power as a curved mirror of radius R=2f, so the effective 

radius of curvature for a resonator which has only a thermal lens stabilizing it is 

For a 400 mW pump, focussed into a wp=lOO mm spot, with 25% conversion to heat, the 

effective radius of curvature is 1.12 meters. Substituting into Eq (l), we find that the spot radius 

of the mode, Wm, for this resonator is given by 

Q = 2 ‘il~ P/X Wp2 (3) 

(4) 

R = 2 k X wP2/ (71~ P WdT) (5) 

31 
wm2 = h wp IT( L k / (X r\~ P dddt) ) / n (6) 

This equation shows that the mode radius Wm changes more slowly than the pump spot size wp. 

It is desirable to have wp<wm to avoid transverse modes. If this condition is not met, Eq. (6) 

indicates that it may be possible to achieve it by reducing wp, reducing power P, or increasing 

resonator round trip path length L. Equation (6) can be solved for the power at which Wp=Wm. 

This solution is 

P = L k ( h / w n I2 / ( X r\~ dddV (7) 

This indicates that with the same pump beam geometry, the TE& power is proportional to 

resonator length. Longer resonators may be a means of increasing power. The disadvantage is 

slightly higher cost, and lower tunability, since mode spacing is smaller. 

We built all-flat-surface ring resonators of the highly nonplanar old-style design. These lasers 

performed in a manner consistent with the thermal focussing calculations above. The effective 

radius of curvature of the resonator as a result of thermal lensing was experimentally determined. 

The spot size of the output 1.06~ beam was measured as a function of the power of the incident 

diode pump beam. Solving Eq. (1) for R, we get 

where n is the index of refraction (1.82 for NdYAG), k is the wavelength (1.06 pm), and L is the 

resonator length (1 1.5 mm). 

The inverse of the effective radius of curvature is plotted as a function of diode power in 

Figure 15. The linear relation is in accordance with Eq. (5) up to a pump level of one watt of diode 

power. At this point, it was very difficult to maintain oscillation in the fundamental spatial mode. 

Too much focusing occd, the mode volume decreased, and higher order modes were excited 

by the pump beam. 

R = wO4 (nx/N2 (UL) (8) 

4. Efficiency of flat resonator 

The amount of single-mode 1.06 pm.output is plotted versus the amount of incident diode 

power in Figure 16. The slope efficiency is 5996, compared to the theoretical maximum of 67%, 

the ratio of the output photon energy to the input photon energy. The threshold could not be 

measured exactly, since the resonator itself was a function of the incident pump power. These 

results were made with a monolithic YAG crystal that had a Nd doping concentration of 1.1%. A 

higher doped crystal was also tried, 1.4% Nd, but the efficiency was less; only 45%. The output 

coupler in both cases was about 1.2% transmissive. 

The greatest amount of single-mode 1.06 pm light was obtained by pumping a flat MISER 

crystal with a 2 W diode. 379 mW of TE& light was observed, but the diode was tumed down 

to 947 mW due to its large (100 micron) emitting aperture. 

32 

4 
Effective thermal focusing vs input power 

200 300 400 500 600 700 800 

input laser diode power (in mW) 

Figure 15. Inverse of the effective radius of curvature in an all-flat resonator as a function 

of the input diode power. As the diode power increases, the thermal focusing increases 

linearly, until about the one watt level of input diode power. 

33 
Power In vs. Power Out 

400 I 

L 

300 

200 - 

4 

"0 100 200 300 400 500 600 

Diode Pump Power in mW 

Figure 16. Optical efficiency of the all flat MISER resonators. 59% optical conversion 

efficiency is achieved fram the input diode power to the 1.06 micron, single longitudinal 

mode output power. The pump used was an experimental low divergence Spectra Diode 

Labs diode with a 60 micron stripe width. This was critical to keep oscillation in the 

fundamental spatial mode. 

34 
5. Injection Locking and Chaining 

In order to achieve higher output powers than would be possible with one laser, and to 

provide redundancy, we studied techniques for combining the power of a number of lasers into a 

single beam. A technique which works quite well for a moderate number of lasers is diagrammed 

in Fig. 17. This technique, which we call injection chaining, is only possible when the laser 

resonator is a ring. The output coupling face of a ring can be used as an injection point for light 

hm anothex laser. Light from the other laser is reflected off the face of the RSOM~O~, and after the 

reflection, output from both lasers is co-linear. If the spatial matching between the lasers is good, 

then the power from both lasers is now combined in a single beam, which is diffraction-limited. 

The light will not in general be single-frequency, as the frequency of both lasers will be present. 

This technique of power-summing is practically limited to a few lasers by the excess complexity 

which results otherwise. A fundamental limit is due to the amount of power lost to imperfect 

optics in each stage. The power after N stages is 

where P is the power of each stage and r is the effective transmission from stage to stage. For an 

infiite number of stages, the power is 

Pmax = P/( 1-r) (10) 

Thus it makes no sense to go much beyond (l-r)-l stages. 

pN = P + rP + GP + r3~+ ... rN-1 P (9) 

There is a well-studied technique, known as injection-locking, where a weak "master" laser is 

injected into the resonator of a more powerful "slave" laser. When the frequency of the master 

laser is tuned within a certain range of the free-running slave oscillator, the master laser locks the 

slave laser to its frequency and dominates the subsequent oscillation behavior. Traditionally, the 

power of the master laser is negligible compared to the slave laser, as in the case when a stable, 

low power laser is used to improve and quiet the output of a high power laser. For this study, the 

goal was to obtain high output powers by combining similar lasers of intexmediate power while 

maintaining single-mode output. Therefore, this technique will be referred to as injection chaining. 

When the frequencies of the two lasers fall within a nmw range of each other, locking 

occurs. This locking range is determined by the relative powers of the two lasers. From [SI, 

where fld is the total frequency range over which the lasers are locked together, and a is the cold 

cavity bandwidth. 

0 . 

fbk = 2 a [power of master laser / power of slave IaserllD (1 1) 

In normal injection locking, the power of the master laser is negligible to that of the slave 

laser. In the case of injection chaining, the powers of the two lasers are comparable. The power 

35 
mode 4 m a tchi ng 

laser cavity I I I Nd:YAG slave 

0 laser diode 

4 HTQ.81 p 

$9 combined 1'06 IJ. x- output 

I beams 

I 

I 

v 

Figure 17 shows a basic diagram for injection locking. The output from 

the master laser is fed into the slave laser cavity. When the frequency 

of the injected signal is within a narrow range around the free-running 

frequency of the slave, the master "locks" and controls the subsequent 

oscillation behavior. When the two lasers are Lightwave's monolithic 

nonplanar ring lasers, unidirectional oscillation is established in the 

presence of a strong magnetic field. 

36 
e 

0 

cc 

e 

a 

of the combined output beam should therefore be the sum of the individual lasers. However, there 

are actually fluctuations in power in the locked range. When the two lasers are exactly the same 

frequency (no detuning) there is actually less power than the sum of the individual lasers. This is 

due to the master laser being resonant in the cavity of the slave laser. When the master laser is 

detuned from the center frequency of the slave laser, there is an increase in power above the simple 

sum due to the saturation in the slave's gain medium by the injected beam. At points outside the 

locking range, the power is simply the sum of the master arid slave lasers. 

The experimental set-up for injection chaining is shown schematically in Figure 18. The 

master laser, a MISER with a lOOmW output, is mode matched with a 135mm lens AR coated for 

1.06 pn into the slave laser cavity. This cavity is pumped with an experimental 500 mW SDL 

laser diode. The laser diode is temperature stabilized to control its wavelength. The monolithic 

crystal of the master laser has a piezo-electric element bonded to the top surface (as described 

previously in the local oscillator section of this report). With voltage applied across this 

transducer, the laser can be quickly tuned. The beam of the master laser is reflected off of a 

beamsplitter that is highly reflective at 450 for 1.06 pm and highly transmissive for 0.81 p. The 

beams from the two lasers are carefully aligned for the best spatial overlap and detected on a silicon 

photodiode. The slave laser is temperature controlled to keep its wavelength stable, and the master 

laser's frequency is tuned until beat notes can be observed on the oscilloscope. As the master laser 

frequency is tuned even closer to that of the slave, the beat notes suddenly disappear as the 

difference frequency falls within the locking range. A completely flat DC signal is observed while 

the master laser is tuned in frequency through the entire locking range. Figure 19 shows the 

locking range sandwiched between unlocked regions, as a 60 Hz sine wave (about 40 volts peakto-peak) is applied to the piezo-electric tuner. The master laser is being tuned more than the total 

locking range, and so the beat frequencies characteristic of unlocked regions are visible (they 

appear as thick white areas of the oscilloscope trace). The varying height of the unlocked regions 

is an indicator of the detection system response. The changing DC level of the locked regions 

shows the power variation with detuning as described in the previous paragraph. 

The locking range can be measured by correlating the voltage needed to scan the entire locking 

range with the tuning coefficient (in this case, about 1 MHz per volt) of the piezo-tuned laser. This 

locking frequency range is plotted versus the square mot of the ratio of the powers in Figure 20. 

These experimental results followed the same slope as the theoretical curve also shown on the 

graph. The deviation of the data from theory is due to incomplete coupling of the two cavities. 

Less power from the master laser is injected into the slave resonator than is theoretically possible, 

and so the master laser appears to be of a lower power rating. Since the locking frequency is 

proportional to the square root of the power of the master laser, the locking range depends on how 

well mode matched and aligned the two cavities are. 

37 
matching 

lens 

slave 

master 

oscillator 

diode 

Pump 

"I V 

I jlRCamera monitor 

diode 

Pump 

power 

meter 

4 

Figure 18. Experimental set-up for injection chaining. With this arrangement, the power 

output, mode quality, and locking range can be monitored simultaneously. 

38 
e 

e 

e 

c 

c 

ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 

2 mddiv 

Figure 19. The injection locked regions can be seen as lines sandwiched between the thick 

white areas which are the unlocked beat frequencies. To obtain this photograph, the 

master laser was scanned at 60 Hz over a range of frequencies greater than the locking 

range. A 40 volt peak-to-peak sine wave was applied to the piezo-elecmc tuner on the 

masterlaser. 

39 
Locking Range vs. Field Strength Ratio 

30 

n 20 

N 

I 

I 

v 

Q) 

CID 

c 

E 

2 

3 

0 

- 0 

10 

0.0 0.1 0.2 0.3 0.4 0.5 

power of slave laser 

Figure 20. Locking frequency range versus the square root of the ratio of the master laser 

power to the slave laser power. Greater locking ranges are obtained with higher injected 

power from the master laser. The deviation from theory is due to imperfect mode matching 

between the two lasers and thermal effects. 

40 
0 

a 

Once the lasers are tuned within the locking range, they will remain locked for minutes at a 

time without any active feedback. Eventually, the two lasers will drift apart, since the usual drift 

rate is about 50 MHz per hour. Investigation into possible techniques for actively locking the 

lasers together was also undertaken. This revealed a promising discriminant to provide a 

correction signal to the master laser. The set-up for this experiment is shown in Figure 21. The 

combined output of the two lasers passes through a half-wave plate and then reflected off of a 

microscope slide placed approximately at Brewstex's angle. The reflected beam, which is now 

mostly vertically polarized, passes through a quarter-wave plate which is set to rotate the 

polarization by 450. The polarization is now 450 to the axis of a polarizing beamsplitter cube 

which the beam then passes through, splitting the beam into two orthogonal polarizations. The 

two beams are then detected on matched silicon photodiodes. One of the signals is inverted and 

then the sum of the two is displayed on an oscilloscope. Scanning the frequency of the master 

laser by applying a sine wave to the piezo-electric tuning element results in a highly dispersive 

waveform that can be seen on the oscilloscope trace in Figure 22. The "folding" at the top and 

bottom of the waveform is due to saturation of the detectors. This discriminant is based on the 

phase change of the master laser beam on reflection from the slave cavity while in resonance [9]. 

6. Modular laser delivered to NASA 

Two high-power laser modules were delivered to NASA in June 1989. Both units of the pair 

contain all-flat highly-nonplanar resonators pumped by a single laser diode. The laser diode is a 

developmental type produced by Spectra-Diode Laboratories of San Jose, California The diode is 

specified as S8106. This diode has an emitting aperture of 60 pm and a divergence of 30'. Both 

of these numbers are very low for a high-power diode, and that is good. Low emitting aperture 

and low divergence make it much easier to focus the light to a small spot. This type of laser diode 

can be operated at 500 mW output for short times, but lifetime is shortened for powers greater than 

300 mW. 

These low-divergence diode pumps led to extremely efficient diode-pumped lasers. We have 

never seen efficiency greater than 40% with any other type of diode, and we achieved 59% using 

the lowdivergence type. The high power and small spot allows us to build lasers which are many 

times above threshold, and which absorb the pump power totally inside the TE& mode. Both of 

these factors lead to high efficiency. 

A problem with this type of laser diode is the high level of amplitude noise. This amplitude 

noise in the laser diode leads to a high level of relaxation oscillation noise in the diode-pumped 

laser. The highly efficient diode-pumped lasers built with these diodes have RMS amplitude 

fluctuations of 3.4% compared to a typical level of 0.3% in the standard diode pumps we use for 

41 
master 

laser 

quarter-wave plate I 

slave 

laser 

1 

I oscilloscope i- photodiode 

plate 

Figure 21. Set-up for creating a discriminant with which to actively lock the two 

lasers together, based on a phase change between the two lasers. 

4 

4 

42 
* 

0 

ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 

Figure 22. Highly dispersive waveform resulting from the difference in phases of the two 

lasers during the injection locked region. 

43 a 
commercial products. Whether or not this noise is a serious problem depends on how the laser is 

used. The diode manufacturer is working to reduce the noise. We may try to use our noise 

reduction electonics to reduce the noise, because the efficiency of these lasers is so good. 

The two delivered modules are package in an injection-chained configuration. The first, or 

master, laser is of the same design as Lightwave's "Model 120-03" 40-mW laser. Figure 23 is a 

photo of the delivered laser. The diode laser power supply is set for maximum output from the 

laser diode of 425 mW. At this level of pump power the output from the YAG laser is 170 mW. 

This high level of power should not be used for more than a few minutes at a time. The longlifetime level of power from the diode is 325 mW. At this level of pumping, the output is 135 

The second, or slave, laser has only one design difference Erom the master laser. The slave 

has an input window and lens as well as an output window and lens. This "input port" is exactly 

equivalent to an "output port" designed for the ring laser when it is lasing in the wrong way. In 

fact, the input port was aligned by forcing the laser to run backwards and aligning components to 

thatbackwardbeam. 

' mW. 

~ 

The assembly containing both lasers has an adjustable mirror which is used to direct the beam 

from the master laser into the input port of the slave laser. This beam strikes the laser resonator 

and is directed through the output port of the slave laser. The transmission from the output of the 

master to the output of the slave is 93%. This is the fraction of the master laser which will be 

added to the power of the slave when it is turned on. 

The maximum combined power from master and slave, with both pumps putting out 425 mW, 

is 340 mW. With the pump power turned down to a long-lifetime level, the combined output is 

260 mW. The optics included in the delivered system assure that the combined output of the two 

lasers is m. The output will also be single frequency if the two lasers are frequency-matched 

to within 40 MHz. This matching is possible by adjusting the precision temperature controller on 

each laser. The temperature controller is stable enough that locked, single-frequency operation is 

possible for 75 minutes without adjustment. After longer time, the two lasers drift out of lock,and 

the frequency difference between the two lasers is observed 

4 

44 
ORlGlNAC PAGE 

BLACK AND WHITE PHOTOGRAPH 

Figure 23. High power laser unit delivered to NASA JPL. The total output power reached 

340 mW. 

45 
V. REMD (Resonant External Modulation and Doubling) 

1. Mechanical Analogy 

Resonant external modulation and doubling can be illustrated with the help of an analogy. In 

Figure 24, a small continuously-operating motor turns a flywheel. The flywheel is designed for 

very low mechanical loss, so a large kinetic energy can be stored. A second wheel can be either 

engaged with the flywheel, or disengaged. It is clear that if the second wheel is usually 

disengaged, then upon engagement large peak powers, greatly exceeding the continuous power of 

the motor, can be transferred to the second wheel, and utilized The continuously-running motor is 

utilized efficiently, since its power is steadily being transferred to the low-loss flywheel. Large 

powers may be extracted from the flywheel at random intervals with precise timing control, and 

there is no need fur a large motor capable of generating the large power directly. 

In the optical system we call a Resonant External Modulator and Doubler (REMD) we use a 

cw single-frequency laser (analagous to the motor,) a large resonant cavity external to the laser 

(analagous to the flywheel,) and a second resonator which is both nonlinear and electro-optically 

tunable (analagous to the second wheel.) The large cavity is held in resonance with the laser, so 

that the circulating power is over 100 times greater than the laser output. Usually the second 

resonator is out of resoance with the first, so there is low power circulating in it, and no 

conversion to the second harmonic. When a pulse is desired, the second resonator is electrooptically tuned into resonance with the first cavity. and large power builds up in the second 

resonator. This large power results in some of the power being converted to green. This power is 

not held in the cavity, but is transmitted. The green pulses may have a peak power well above the 

power of the 1.06-jm cw laser. 

2. Possible Configurations 

Two configurations for a REMD system were considered. A linear and a ring configuration 

are shown in Figure 25. The linear configuration is easier to align, but an optical isolator is 

necessary to avoid destabilizations from feedback. The ring configuration avoids the feedback 

problem, but involves more optics and alignment time. The laser must be mode matched into the 

large cavity, which must be mode matched to the small cavity. For this experiment, a ring 

configuration was chosen. Instead of a small second cavity composed of discrete optical 

components, a monolithic ring resonator was used. This cavity is a solid piece of magnesiumdoped lithium niobate, fabricated with two total internal reflections. The front curved surface is 

1 

46 
e 

* 

I, 

c 

c, 

Motor ~ 

lrce to engage 

disengage 

Fig. 24. Mechanical analog of resonant external modulator / doubler. 

Motor continuously powers flywheeL Large peak power can be 

extracted when movable wheel is occasionally engaged with flywheel. 

47 
Figure 25 depicts two possible configurations for resonant external modulation 

and doubling. The first layout has two linear, standing wave cavities. The 

first cavity consists of the curved mirror next to the laser and the flat mirror 

on the left of the frequency doubling crystal. The second cavity contains the 

nonlinear material and is bounded on the right by a curved mirror which is 

highly reflective for 1.06 p light and transmissive for 532 nm light. 

large empty cavity "stores" energy, which is then "dumped" quickly to the 

smaller ring cavity when brought into resonance. 

The lower configuration consists of two traveling wave ring cavities. The 

48 
0 

0 

* 

0 

polished and coated to be 98% reflective at 1.06 pm. The top and bottom surfaces of this crystal 

are gold plated to allow electrodes to be attached to it, as shown in Figure 26. The crystal was held 

in between copper plates with leads soldered on. Electric fields can now be applied across the 

crystal, to induce changes in the refractive index via the electro-optic effect. This changes the 

effective length of the crystal. The laser frequency is tuned to resonate in the large cavity, so large 

circulating powers build up. The small cavity stays off resonance during this time, so its internal 

fields small. When a pulse is desired, the sd cavity is quickly switched into resonance with 

the right voltage. The large circulating power in the large cavity is now suddenly made available 

(dumped) to the small cavity, and an intense green pulse is emitted. 

3. Experimental Set-up 

As diagrammed in Figure 27, a 50 mW laser is mode matched with a 7.5 cm focal length, AR 

coated lens, into a large cavity, consisting of a 98% reflecting flat mirror, the coated, curved 

surface of the doubling crystal, and an HR mirror with a 40 cm focal length. The round-trip length 

of the cavity is about 79 cm. The doubling crystal's hnt surface has a radius of curvature of 26 

mm and a round-trip path length of 25 mm. 

Instead of designing the electronics to lock the laser frequency to the large cavity, a simpler 

approach was taken. The HR 40 cm mirror was mounted on a piemelectric stack, which was 

scanned slowly at 60 Hz to bring the cavity in and out of resonance with the laser. The doubling 

crystal was quickly switched with a function generator to scan the small cavity through resonance 

at a fast rate compared to the large cavity. pulse trains of green light were detected, corresponding 

to peak powers of 50 mW of 532 nm light. The envelope of the pulses is the 60 Hz cycle of the 

large cavity resonance with the laser ftequency. The lithium niobate ring resonator (the small 

cavity) was tuned at about 300 kHz over a 30 Volt range, giving 2% of a free spectral range of 

tuning, or 7.2' of modulation, if a full fkee spectral range is 360". 

c 

Figure 26. The monolithic, lithium niobate ring resonator used as the second cavity in the 

resonant external modulation and doubling technique. 

49 ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 
oscilloscope 

photodiode 

IR blockfilter 

laser 17 

piezo -electric stack 

HR @ 1.06 pm 

1.06 pm 

Figure 27 shows the experimental set-up for resonant external modulation and doubling. 

A 40 mW MISER laser is mode matched into a large cavity consisting of a 98% 

reflecting flat mirror, a 40 cm focal length high-reflecting mirror, and the front curved 

surface of a magnesiumdoped lithium niobate resonant doubling crystal. This 

monolithic non-linear crystal is the second resonator, consisting of the front coated 

surface, and two total internal reflections. It has electrodes attached to the top and 

bottom surfaces, so its index of refraction (and effectively, its length) can be changed via 

the electro-optic effect. When the MISER is resonant with the big cavity and not the 

small one, high circulating powers build up in the large cavity. Then, the small 

monolithic cavity is quickly switched into resonance with the large cavity. The stored 

energy is then "dumped" into the non-linear crystal, and a powerful green pulse results. 

50 
e 4. Computer model 

0 

e 

A computer program was written which modelled the REMD process. A more complete 

description of this model is given in Appendix 2. This model is a one-dimensional finite-element 

model. The program divides the two resonators of the REMD system into a few hundred 

elements. This division is along the direction of propagation of the beams - the transverse field 

distribution is assumed to be Gaussian. The amplitude and phase of the light in each element is 

stored. The program works by keeping track of what happens to light in each element in the few 

picoseconds which it takes to transit through the element. In most elements, nothing happens - the 

light is simply translated into the next element. In the electmoptic element, the light is either phase 

shifted or not, depending on whether or not the second cavity is in or out of phase with the first. 

In the doubling element, some green light is created, proportional to the square of the infared 

power. The infrared power is reduced by an amount which conserves energy. The two points 

where the most happens are where the beam from the laser enters the fmt cavity, and where the 

two cavities meet. At these points the complex amplitudes of the two beams aree combined 

according to the reflectivity of the hr. Losses are also accounted for at the hrs. 

The parameters which define the particular REMD system being modelled are entered from a 

menu. This menu is shown in Table 5, along with the definition of each parameter. The values 

shown are for a hypothetical wellengineered system pumped by a 1-Watt laser. 

The program begins with the steady-state solution stored in each element. The system is 

assumed to have been in the "no transmission" mode for a long time. Once the program begins, 

the power in each element is calculated as a function of time. The output of the program is the 

power of the green light transmitted from the second cavity, as a function of time. 

This program, and a companion program which did a crude information analysis of the 

transmitted green pulses, were delivered to NASA. More details of these programs are found in 

Appendix 2. 

The computer model was used to estimate the peformance of REMD systems which we 

thought wm achievable, and also to model the system we actually built. Figure 28 shows the 

pulse train transmitted from a system pumped with a 1 Watt pump. The large ratio of peak power 

transmitted to cw input power is apparent. 

51 
201.0000 A: 

12.0000 B: 

99.0000 c: 

80.0000 D: 

0.3000 E: 

1.0000 F: 

97.0000 G: 

0.2500 H: 

1.0000 I: 

90.0000 J: 

1.0007 K: 

1.5610 L: 

TABLE 5 

Parameters Defining Resonator: 

Length of 1st resonator half-round-trip, mm. 

Length of 2nd resonator half-round-trip, mm. 

Reflectivity of input mirror, %. 

Reflectivity of coupling mirror, %. 

Loss in first cavity, %. 

Loss in second cavity, %. 

Coupling efficiency, % of transmitted light. 

Nonlinear coefficient, % / Watt. 

Parameters Defining System Inputs: 

Input power, Watts. 

Modulation phase, degrees. 

In-phase time period, nsec. 

Length of single time slot, nsec 

Parameters Defining Calculation Format: 

0.3903 M: Period between data outputs, nsec. 

3.0000 N: Length of elemental light slice, mm. 

Files for I/O: 

REMD. IFL 0. Access file for data stream 

REMD. OFL P. Output file, transmitted light 

REMD. MFL Q. Name of this master file 

1 

1 
0 

70 

60 

50 

40 

30 

0 

0 

RESONANT MODULATION AND DOUBLING 

Example Case REMD.QFL a0 I 

I b- - - 

I I I I I I I I I I I 

0 20 40 60 80 100 120 140 160 

lime, nSEC 

180 200 

Figure 28. Thmmical pulse train transmitted from a REMD system pumped with a one 

watt laser. 

8 

53 
5. Verification of program results with bench-top results 

The system we actually built did not have a 1-Watt pump, and many other aspects of the 

system were not what could be achieved in an engineered system. We built it mostly to verify the 

computer model. The experimental results agreed with those predicted by the computer model. 

Peak green pulses of 50 mW were obtained with this set-up. Oscilloscope traces of a single pulse 

and the pulse train are shown in Figure 29. The factor limitting the peak power of the green pulses 

was the rate at which the second, small cavity could be switched into resonance. Higher powers 

were achieved as the switching time decreased, until the point when the speed was instrument 

limited. The full width of the green pulse (at the half-maximum point) was measured to be less 

than 13 nanoseconds. Higher peak power pulses corresponded to thinner pulses. The computer 

model predicted peak powers of around 45 mW of green light in a 15 nanosecond pulse. The 

values of the input parameters for this case are shown in Table 6. Some values, such as the 

reflectivities of the mirrors and the cavity lengths, can be determined in a straightforward manner. 

Others, however, such as the coupling between the two cavities and the in-phase time period, are 

not easily measured. Coupling into the large cavity by the laser is not taken into account with this 

model, and so the initial input power, Parameter I, was reduced to the amount that was estimated to 

mode-match well into the large resonator. This value was estimated at about 50% of the initial 

laser power, or 25 mW. The two cavities were not perfectly matched, and so Parameter G was set 

at 60%. Parameter J, the electro-optic phase shift, was set at 100. This is equal to the ratio of the 

voltage applied to the doubling crystal compared to the voltage needed to tune the crystal over a full 

free spectral range. The model assumes that instantaneous switching into resonance occurred, 

while the best that could be achieved experimentally was only 120 nanoseconds. All these 

uncertainties mean that a precise quantitative test of the model was not possible. Nevertheless, we 

are confident that the model is useful for factor-of-two estimates of system performance. 

Figure 30 is a photo of the benchtop experiment used to veriEy the REMD model. 

54 
0 

e 

0 

ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 

.. - - 

Figure 29. Oscilloscope traces of a pulse train and a single pulse from the experimental 

REMD system. Peak power at 532 nm reached 50 mW with a FWHM of 30 nsec. This 

set-up used a 50 mW 1.06 pm laser as a pump, but only 25 mW was actually coupled into 

the REMD system. 

55 
TABLE 6 

Parameters Defining Resonator: 

Length of 1st resonator half-round-trip, mm. 

Length of 2nd resonator half-round-trip, mm. 

Reflectivity of input mirror, %. 

Reflectivity of coupling mirror, %. 

Loss in first cavity, %. 

Loss in second cavity, %. 

Coupling efficiency, % of transmitted light. 

Nonlinear coefficient, % / Watt. 

380.0000 

25.0000 

99.0000 

98.0000 

0.5000 

3.0000 

60.0000 

0.1000 

A: 

B: 

C: 

D: 

E: 

F: 

G: 

H: 

Parameters Defining System Inputs: 

0.0250 I: Input power, Watts. 

10.0000 J: Modulation phase, degrees. 

150.0000 K: In-phase time period, nsec. 

150.0000 L: Length of single time slot, nsec 

Parameters Defining Calculation Format: 

0.4003 M: Period between data outputs, nsec. 

5.0000 N: Length of elemental light slice, mm. 

Files for I/O: 

DATA ENTRY BY HAND. 0. Access file for data stream 

REMD. OFL P. Output file, transmitted light 

remd. mf 1 Q. Name of this master file 

1 
e ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 

Figure 30. Experimental set-up for resonant external modulation and doubling. 

A 50 mW laser, seen in the lower left hand comer of the photograph, is mode 

matched with the lens in the lower center of the photograph, into a large empty 

cavity. This cavity consists of the 98% reflecting flat mirror (lower right hand 

corner, at a 450 angle), the front surface of a monolithic doubling crystal (white 

square immediately to right of the flat mirror), and the 40 cm focal length curved 

mirror in the upper right hand corner. The monolithic doubling crystal is a solid 

piece of lithium niobate which serves as the second resonant cavity, where 

frequency doubling occurs. Voltage is applied to the crystal (via electrodes 

attached to the gold plating on the top and bottom of the crystal) to change the 

optical cavity length of the resonator. The 40 cm focal length curved mirror is 

mounted on a piezo stack so the length of the large cavity can also be 

modulated. 

57 
VI. Relation to Future Work 

1. Increasing power output 

We demonstrated that a power of 310 mW cou e generated, in a ffraction-limited, sing efrequency beam, pumped by a single laser diode. This was limited by the appearance of higherorder transverse modes, due to mismatch between the fixed size of the pump beam and the size of 

the lasing mode, which diminished as pumping increased. There are three ways to reach larger 

power levels, while maintaining the advantages of the monolithic ring design. The first is to use 

the "injection chaining" technique developed under this contract. This allows the coherent 

summation of several lasers, at some cost in complexity. This greater complexity can also lead to a 

desirable redundancy if the system is well designed. The second way to increase power is to 

increase the length of the resonator, while maintaining the same pump beam. This will allow 

diffraction-limited power to increase. According to Eq [7] the increase will be proportional to 

cavity length. The third way to incnxse power is to use smaller pump beams with larger power 

levels. This will be possible if there are improvements in diode technology, or through the use of 

techniques to combine beams through polarization or spatial multiplexing. With these techniques it 

should be possible to go beyond 1 Watt. The 10-Watt level will not be possible without more 

fundamental changes of design. 

1 

2. Modulation of output 

To transmit information on the laser beam, techniques for either phase modulation, 

conventional binary amplitude modulation, or pulse-position modulation must be developed. 

There are conventional techniques for doing the first two with bulk or guided-wave optics. There 

are problems with these techniques involving either optical damage (for the guided-wave 

modulators) ore high speed modulation of high voltage (for bulk modulators.) We have 

demonstrated a novel technique for PPM, but it is rather elaborate. 

We believe that there are techniques for both phase and amplitude modulation that require 

neither high voltages or guided waves. These techniques take advantage of the multiplicative 

effects which occur when a cavity is nearly resonant with a highly coherent laser beam. The phase 

of a beam reflected from the cavity, or the transmission through the cavity, will change very 

quickly as a function of the cavity length, which is controlled by a voltage. We hope to prove the 

feasibility of these techniques shortly. 

58 
3. Untended operation 

e 

0 

Laser diodes have a structure of mode-hops which is much more complicated than that of the 

YAG laser they pump. The temperature of the diode laser is adjusted to compensate for the mode 

hops. The individual making the adjustment must find a temperature where the power of the 

diode-pumped laser is adequate and the noise of the diode-pumped laser is suitably low. Both 

these conditions must hold over a range of adjacent tempemtures, or the laser will drift to a point of 

lower power or higher noise. Currently these adjustments are made by technicians at Lightwave. 

If component values, or the laser diode itself, drift, then these adjustments may have to be 

repeated. 

In an untended system, the parameters of power and noise would have to be monitored by a 

computer, and adjustments to temprature to counter aging also made by a computer. In principle, a 

computer could make these adjustments much more consistently than a human. We believe 

devlopment and testing of a system for untended operation of laser diode-pumped lasers is 

warranted. 

4. Diode lifetime and reliability 

e 

e 

In principle, laser diodes can be built to last over 100,OOO hours. The diodes that Lightwave 

uses are expected to last 10,ooO hours. Our experience is that many diodes last this long with only 

a small reduction in power, while others die prematurely. A program aimed at long-lifetime 

diodes, and also on means to Screen diode batches to reduce early death, is important. This type of 

work is already going on at diode manufacturers laboratories. 

VII. Conclusions 

The local oscillator laser we built has the power, frequency stability, amplitude stability, and 

tunability needed for a local oscillator in a coherent communication system. The kilohertz 

linewidth of these lasers will permit coherent detection techniques to work even at very low bit 

rates. The demonstration of phase-locking makes possible the detection of phase-modulated light 

with maximum sensitivity. The high power and shot-noise-limited amplitude stability of the local 

oscillator will result in heterodyne detection systems with quantum-noise-limited sensitivity. 

We built a single-frequency laser with an output power of 310 mW. We also demonstrated a 

technique of combining the power of multiple lasers in a way that can preserve the diffractionlimited, single-mode character of the individual modules. The pair of modules delivered to NASA 

59 
have a combined output of 340 mW. This combining technique, which we call injection chaining, 

will permit us to reach powers above 1 Watt, while building systems with some redundancy. 

We studied a technique, which we call Resonant External Modulation and Doubling (REMD), 

for converting the cw, infrared output fn>m single-frequency lasers into randomly pulsed, visible 

output suitable for PPM transmission. A computer model was written and delivered to NASA, and 

benchtop experiments were used to verify the model. Both model and experiments confirm the 

potential of this approach. The REMD technique is somewhat complicated, and the experiments 

carried out were at low power. More work would be necessary to prove that this technique is 

workable at high efficiency, and to engineer the technique for reliable performance. 

VIII. References 

d 

4 

1. T. J. Kane and R. L. Byer, "Monolithic, unidirectional single-mode Nd:YAG ring laser," 

Opt. Lett., vol. 10, p 65, 1985. 

nonplanar ring oscillators," J. Quannun Electronics, vol. 25, p. 767, 1989. 

2. A. C. Nilsson, E. K. Gustafson, and R. L. Byer, "Eigenpolarization theory of monolithic 

3. W. Koechner, "Solid-state Laser Engineering," First Ed. Springer-Verlag, 1976, p. 228. 

4. U. S. Patent #4,829,532, "Piezo-electrically tuned optical resonator and laser using same." 

5. T. J. Kane and E. A. P. Cheng, "Fast frequency tuning and phase locking of diode6. I W. Smith, "Optical resonator axis stability and and instability from first principles," Fiber 

7. W. Koechner, ibid., p 353, Eq. (7.24). 

8. A. E. Siegman, Lasers, University Science Books, Mill Valley, CAY 1986. 

9. T. W. Hansch and B. Couillaud, "Laser frequency stabilization by polarization 

spectroscopy of a reflecting reference cavity," Opt. Comm., vol. 35, p. 441,1980. 

issued to T. J. Kane, April 1989. 

pumped Nd:YAG ring lasers," Opt. Lett., vol. 13, p 970, 1988. 

Optic and Laser Sensors, Proc. SPIE, vol. 412, p 203, 1983. 

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