NASA Contractor Report 3341 

Single-Mode Laser Studies: 

(I) Design and Performance of 

NASA 

CX 

3341 

c. 1 

Fixed- Wavelength Laser Source and 

(II) Coupling of Lasers to 

Thin-Film Optical Waveguides 

I. Ladany and J. M. Hammer 

CONTRACT NASl-I 5440 

OCTOBER 1980 
NASA Contractor Report 3341 

TECH LIERARY KAFB, NM 

Single-Mode Laser Studies: 

(I) Design and Performance of a 

Fixed-Wavelength Laser Source and 

(II) Coupling of Lasers to 

Thin-Film Optical Waveguides 

I. Ladany and J. M. Hammer 

RCA Laboratories 

Primeton, New Jersey 

Prepared for 

Langley Research Center 

under Contract NASl-15440 

NASA 

National Aeronautics 

and Space Administration 

Scientific and Technical 

Information Branch 

1980 

i 

.- 

A 

- 

PREFACE 

This final report covers work done at RCA Laboratories during the 

period July 3, 1979 to April 3, 1980 under contract No. NASl-15440. 

This work was performed in the Solid State Devices Laboratory, under 

the direction of B. Hershenov. The Group Head was M. E. Ettenberg and 

the Project Scientist was I. Ladany. Part I of this report was written 

by I. Ladany and Part II by J. M. Hammer. Staff members and support 

personnel who contributed to this work in addition to the authors, and 

the area of their contribution, are listed below: 

D. Botez;: 

L. Elbaum 

D. Gilbert 

M. Harvey 

C. C. Neal 

J. 0. Schroeder;: - 

Single-mode lasers 

Fiber coupling 

Spectral measurements, temperaturedependence studies 

Device fabrication 

Coupling measurements, lens and prism mount 

design 

Design and construction of temperaturecontrol circuits 

*Members of Technical Staff. 

iii 

TABLE OF CONTENTS 

Section Page 

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

I. 

II. 

DESIGN AND PERFORMANCE OF A FIXED-WAVELENGTH LASER 

SOURCE MODULE ......................................... 

A. Objective ......................................... 

B. Module Design ..................................... 

1. Temperature Control Circuit ................... 

2. Laser Driver .................................. 

C. Theoretical Discussion ............................ 

D. Experimental Results .............................. 

1. Temperature-Induced Wavelength Changes ........ 

2. Output Fluctuations Due to Temperature Cycling. 

3. Optical Feedback Effects ...................... 

COUPLING OF SINGLE-MODE LASERS TO THIN-FILM OPTICAL 

WAVEGUIDES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

A. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

B. Selective Grating Coupling . . . . . . . . . . . . . . . . . . . . . . . . 

C. Miniature Cylindrical Lens System for Collecting, 

Symmetrizing, and Focusing Junction Laser Beams . . . 

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

2. Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

D. Prism Coupling to Optical Waveguides . . . . . . . . . . . . . . 

E. Experimental Observations and Device Construction . 

III. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 

1 

3 

3 

4 

4 

8 

8 

12 

12 

17 

20 

23 

23 

25 

32 

32 

33 

40 

43 

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 

V 
LIST OF ILLUSTRATIONS 

Figure Page 

1. Details of the CDH structure ........................... 2 

2. Photograph of the PC board showing parts layout ........ 5 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

Temperature control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

(a) Photograph of interior of complete unit . . . . . . . . . . . . 

(b) Exterior of complete unit . . . . . . . . . . . . . . . . . . . . . . . . . . 

Driver IC functional diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Wavelength shift vs temperature . . . . . . . . . . . . . . . . . . . . . . . . 

Wavelength dependence on duty cycle, CDH laser . . . . . . . . . 

Output spikes due to TE pulsing . . . . . . . . . . . . . . . . . . . . . . . . 

Comparison of fiber output with temperature of the MCB. 

(a) Laser driven with separate power supply, (b) laser 

driven with internal IC, and (c) temperature of MCB . . . . 

Laser output (measured at fiber end) for 19 and 25OC . . . 

Optical feedback effects: (a) is output from laser 

before coupling; (b) aft er coupling to graded index 

fiber having the input end beaded as in photomicrograph; 

(c) same as (b) except that the end of the fiber 

(away from laser) was immersed in high index fluid; 

(d) output from fiber with more rounded input end . . . . . . 

Thin-film waveguide and corrugated grating coupler. 

The amplitude of the grating is exaggerated . . . . . . . . . . . . 

Selectivity of waveguide grating coupler. The grating 

space and angle are chosen to optimize the coupling 

at A 0’ IO is the intensity coupled at Ao. I the 

intensity coupled at some other wavelength A . . . . . . . . . . . 

6 

7 

7 

9 

14 

15 

17 

18 

19 

21 

26 

28 

vi 
LIST OF ILLUSTRATIONS (Continued) 

Figure 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

Traces of the spectral output. Lower trace: directly 

from the junction laser; upper trace: light grating 

coupling into LNT waveguide and prism coupled out of 

waveguide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Perspective illustration of cylinder lens system for 

symmetrizing junction laser output . . . . . . . . . . . . . . . . . . . . . 

Ray diagram for calculating cylinder lens parameters 

to symmetrize junction laser output. The lateral 

direction is shown by dashed lines and has been 

rotated 90° around the Z axis . . . . . . . . . . . . . . . . . . . . . . . . . . 

Plot of DL/DT and AL/DT as a function of normalized 

beam waist W/ho. The lens output "convergence" angle 

01 is shown on the upper scale. The refractive index 

of the transverse lens n T is the parameter . . . . . . . . . . . . . 

Plot of G/DT and S/O, vs W/ho; nT is the parameter. 

When G < 0, the lenses overlap. When G/G n > 1, not all 

of the laser light is collected at n = 1.8 G/DT < 0 

(negative) when W/ho % 2.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Schematic diagram of a waveguide-prism film coupler . . . . 

Sketch of an improved cylinder lens holder. The slots 

are spaced so that the gap is zero. Shims are used to 

give the desired gap and the cylinders are cemented 

into place . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Experimental x-y trace of the lateral and transverse 

beam waist. The center figure is on x-y tracing of 

the far-field pattern of light coupled out of the 

LNT guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

22. Schematic of laser-cylinder lens LNT waveguide assembly. 

vii 

Page 

31 

33 

34 

39 

40 

41 

44 

45 

46 
LIST OF ILLUSTRATIONS (Continued) 

Figure Page 

23. Cross-section of mechanical module showing details of 

laser-lens-LNT waveguide mount . . . . . . . . . . . . . . . . . . . . . . . . . 46 

24. Photograph of final laser-lens-waveguide module . . . . . . . . 48 

25. Photograph of laser-lens-waveguide module in protective 

plastic case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 

26. Plot of laser power vs laser drive current and spectrum 

of laser output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 

27. Plot of light intensity output of lens system vs laser 

drive current and spectrum of lens system output. The 

input is that shown in Fig. 26 . . . . . . . . . . . . . . . . . . . . . . . . . 50 

28. Plot of light intensity output coupled out of LNT guide 

of module vs laser current and spectrum of same. The 

input is that shown in Fig. 27 . . . . . . . . . . . . . . . . . . . . . . . . . 51 

viii 
INTRODUCTION 

Various schemes have been developed in recent years for obtaining 

single-mode injection lasers. The most practical devices of this type 

seem to be those in which lateral mode confinement is built into the 

laser structure. RCA's version of this device, called the CDH 

(Constricted Double Heterostructure) laser, is illustrated in Fig. 1. 

The lateral confinement is obtained by introducing a variation in 

the cavity thickness, which leads to an index of refraction variation 

in the plane of the junction. This index variation forms a fixed 

waveguide which, when properly designed, supports only a fundamental 

lateral mode. The transverse mode which lies in the plane perpendicular to the junction plane is also supported by a fixed waveguide and 

is constrained to be fundamental by the index changes at the (AlGa)As 

heterojunctions and the thickness of the active region. The longitudinal or spectral mode is fixed by the cleaved mirror spacing and 

there is no built-in selection as there is for the spatial modes 

(transverse and lateral). For the CDH laser, in common with other 

lasers of this index-guided type, if the Fabry-Perot cavity is kept 

short and the material quality high, one longitudinal mode dominates 

over a considerable current range. In this sense, such a laser is 

called a single-mode laser. 

There have not been too many practical applications for singlemode lasers described in the literature. For highest data rates and 

longest fiber links such lasers are expected to be of importance, 

but there are other interesting applications where single-mode lasers 

will also prove useful. 

In the present report we discuss one such application, the utilization of single-mode lasers as fixed-wavelength sources to be used 

to provide wavelength multiplexing capability in optical-fiber data 

links. In order to gain full benefit from the use of such lasers it 
is necessary to couple their radiation into suitable structures. The 

second part of this report covers a study of the coupling of singlemode lasers into thin-film optical waveguides. 

METAL CONTACT 

SiO2 

P+- Go As 

p- (AIGa) As 

IAIGaIAs 

n- (AIGaIAs 

n+ - GaA8 

MISORIENTED SUBSTRATE 

[I-r off (1001 towards (OIO)] 

0 SINGLE-MOOE (IO-l5mW) 

0 REAL- INDEX WAVEGUIDING 

0 T,r200K 

Figure 1. Details of the CDH structure. 

Use of trade names or names of manufacturers in this report does 

not constitute an official endorsement of such products or manufacturers, either expressed or implied, by the National Aeronautics and 

Space Administration. 
I. DESIGN AND PERFORMANCE OF A FIXED-WAVELENGTH 

LASER SOURCE MODULE 

A. OBJECTIVE 

The objective of the first part of this program was to design and 

build temperature-stabilized modules for driving RCA-developed singlemode lasers. In use, four separate modules drive four lasers operating 

at different wavelengths at least 75 8 apart, the lasers being permanently coupled to short lengths of multimode graded-index fibers. 

These modules are intended for a wavelength multiplexing experiment 

under development by NASA and its contractors. While modules to drive 

and stabilize the power output of lasers have been designed previously, 

there has been no previous attempt to prove a working device which 

stabilizes the spectral output of a laser. 

Detailed specifications for the module are listed in Table 1. 

TABLE 2. MODULE SPECIFICATIONS 

Parameter Value 

Overall Size 7.6x7.6 cm or smaller 

Driver Fiber-optic transmitter 

type 2144 

Power Supply + 5 Vdc 

Wavelengths Four separate values, at 

least 75 8 apart, with 

10-g stability against 

temperature changes. 

Power At least 1.5 mW into fiber 

Fiber ITT graded-index, type 201 

Maximum Laser Current 150 mA 
B. MODULE DESIGN 

The design of the driver module was largely dictated by various 

NASA requirements. The overall size was set by the need to incorporate 

a cable connector on one side and a fiber-optic bulkhead connector on 

the other. Furthermore, it was necessary to provide a convenient storage 

method for a 30- to 40-cm length of fiber. It was decided to place all 

the circuits on one board, as shown in Fig. 2. The thermoelectric element (TE) is mounted through the middle of the board, directly to the 

baseplate. Special identification of some terminals has been made, as 

they are used in setting the laser current. 

1. Temperature Control Circuit 

In order to maintain the size and voltage specifications, a transformerless circuit was designed to control the current through the TE, 

and thus the laser temperature. A diagram of this circuit is shown in 

Fig. 3. Temperature is sensed by a thermistor mounted at the base of 

a metal coupling block (MCB) which holds the laser and couples it 

thermally to the TE. The unbalance signal from the thermistor bridge 

is amplified and applied to a pass transistor which finds itself in one 

of two possible states: off or full on. Thus the TE is run with a 

form of off-on circuit and is continuously cycled between these two 

states. The advantage of this circuit is that the transistor is driven 

to a high conducting state and therefore has a minimum voltage drop and 

least sensitivity to temperature fluctuations. In order to succeed in 

this design, it was necessary to match the TE to the transistor in order 

to obtain a joint drop of 5 V. The current into the TE has a fixed 

amplitude of about 1 A, and the power supply should have a rating of at 

least 1.5 A. 

Besides the laser, the MCB also contains a load resistor whose 

purpose is to supply a small amount of heat (l/8 W). This heat drives 
Figure 2. Photograph of the PC board showing parts layout. 

the laser to a temperature above ambient, thus exercising the TE even 

below ambient temperature. Good temperature control around ambient 

is therefore easier to obtain. Without this heater, only temperatures 

5 
+5V IN 

A 

220K 

/,A176 3.5Q~THERuOE~CTRIC r- _ -. -..-..- 

r ----------- 

I CONDITION 

I ” F L I P ” +2.57 +3.75 ; 

I “FLOP” + 2.37 +0.48 I 

L--------------l 

50 SIG. IN 

A N.C. 

BA-Uc +5v 

D GND 

Figure 3. Temperature control circuit. 

above ambient could be stabilized, although self-heating from the laser 

also serves to raise the uncooled operating temperature. An additional 

advantage of the heater is that it reduces the time the laser operates 

at temperatures below the set point, such as might occur in early 

morning start-up. The optimum value for the heat put in by this load 

resistor has not been established, and might very likely turn out to be 

higher than that used. The circuit gain is adjusted to provide control 

within +O.lOC. - However, enough gain is available so that the temperature limits can be reduced by a factor of 10, or more, if desired. A 

photograph of the assembled circuit is shown in Fig. 4(a). 

6 
Figure 4(a 1) * Photograph of interior of complete unit. 

Figure 4(b). Exterior of complete unit. 

7 
2. Laser Driver 

The current to the laser is delivered by an IC developed for the 

Government and supplied to us by NASA. It accepts input TTL pulses and 

delivers current pulses to the laser of up to 150-mA amplitude. The 

unit seems to perform very well, although there are two areas in which 

it could be improved. First, the current setting is awkward since external resistors have to be used for intervals finer than 25 mA, and 

second, the bias is fixed at 50 PA, which is often below the desirable 

threshold bias of 50 to 100 mA. 

An exterior view of the complete module is shown in Fig. 4(b). 

For convenience we provide a summary of the procedure used to adjust 

the laser driving current. (Refer to Fig. 5 for a schematic diagram 

of the IC and to Fig. 2 for terminal locations.) 

Any current up to 150 mA can be had by selecting a resistor whose 

value is given by R = 1.83/I, where R is in ohms and I in amperes, and 

soldering it between terminal 5 and ground. Otherwise, various discrete 

current values can be obtained by selecting one or more of the terminals 

marked 7, 8, and 9 and connecting them to ground. This selects one of 

several possible resistance values and yields a current given by the 

above equation. 

C. THEORETICAL DISCUSSION 

The injection laser is sensitive to temperature, which affects all 

its properties. The wavelength of a laser depends on the energy bandgap of the material and on the cavity which selects the particular 

modes. If, in the course of operation, there is an alteration in 

these properties, for example, if the contacts age, permanent changes 

in the current flow can occur which affect the mode gain or the average 

temperature. This may result in a permanent change in the emission 

wavelength. In general, however, it is possible to eliminate these 
+5vc CER TAN 

f 

1.83 

I 

Figure 5. Driver IC functional diagram. 

factors by proper laser design on the one hand, and by aging (burn-in) 

on the other. In the discussion to follow we will be concerned with 

properly designed and/or aged lasers in which this effect can be 

neglected. 

As is well known, the threshold current of a laser can be characterized by an exponential dependence on temperature of the form Ith = 

IO exp(T/To) where To is typically 100°C to 120°C for AlGaAs devices. 

This threshold sensitivity manifests itself in a large change in output 

as the temperature is changed (to be discussed below). 

A change in output power, however, does not necessarily result in 

a wavelength change because most well-made single-mode lasers maintain 

their wavelength over a substantial range of outputs. Thus, it is not 

so much the change in output power but the change in temperature and 

wavelength itself which is our main concern. 

9 
As mentioned before, a change in bandgap affects the energy released 

in recombination. A common expression for the dependence of bandgap on 

temperature is [l] 

Eg(T) = 1.52 - 5.8 x 1O-4 T2/(T + 300) (1) 

from which we obtain for the change of energy with temperature 

% -5.8 x iom4 [l -(T :";OO)2] % 0.435 meV/'C (2) 

This is the change observed for the spontaneous emission, the laser 

emission being not quite the same because of other factors such as the 

density of states dependence on temperature and band filling. For an 

8200-X laser, the bandgap-induced change is about 2.5 to 3 8/'C. The 

spontaneous emission line is very broad (200 8) so that many Fabry-Perot 

modes can arrange themselves within it, and a slight shift in this spontaneous envelope does not necessarily shift the emission line. Because 

of this and other effects, such as the gain suppression by the oscillating field [2], the bandgap has to change by a minimum amount before the 

wavelength abruptly jumps to a new value. As a rough guide, we may 

assume the wavelength will change when the spontaneous emission shifts 

by an amount equal to one or two Fabry-Perot mode spacings, i.e., 5 to 

10 8 for most of our lasers. Thus the shift may occur for changes in 

temperature of 2 to 4OC. 

Superimposed on this distxntinuous shift is a smooth change in 

wavelength due to a change in cavity length, induced by the thermal 

1. M. B. Panish and H. C. Casey, Jr., J. Appl. Phys. 40, 163 (1969). 

2. M. Yamada and Y. Suematsu, IEEE J. Quantum Electron. QE-15, 743 

(1979). 

10 
expansion of the material and the change in refractive index. These 

effects are conveniently described by [3,4] 

dh -&=A G&+1!& 

L dT n dT 1 (3) 

Since l/L dL/dT * 0.6 x 10e5/"C and l/n dn/dT - 1 x 10m4/"C, the 

index change is the dominant effect, and the wavelength shift is given 

by 

g s 0.82 Spc (4) 

As far as external disturbances are concerned, such as changes in 

ambient temperature, the problem is relatively straightforward. The 

laser is mounted on a TE, and with suitable sensors and control circuits its temperature can be kept constant within any desired limits. 

Much more important is the self-heating effect. The power output of 

a laser is given by 

Po=IVt-j (5) 

where I is the current, V the applied voltage, and Q the light emission 

efficiency of the laser. For typical lasers, q * 0.06 to 0.20, so that 

most of the input power appears as heat. The largest fraction of this 

power dissipation occurs in the laser cavity due to nonradiative recombination. Significant amounts are also released in the contacts, 

and in the bulk material by photon absorption. This heat raises the 

cavity temperature, beginning at the instant current is applied to the 

laser. Thus it is clear that no matter how we cool the laser, its 

temperature will begin to rise as soon as a pulse is applied. If the 

3. M. Cardona, Int. Conf. Semicond. Phys., Prague, 1960, p. 388. 

4. S. I. Novikova, Sov. Phys. Solid State 3, 129 (1961). 

11 
pulse is very short, 100 ns or less, the rise will be very small, and 

if we bias the laser to threshold, the steady current will develop an 

\ 

average temperature in the laser, which will change by a much smaller 

amount as the driving pulse is applied, because junction impedance is 

now low, or what amounts to the same thing, the laser differential 

emission efficiency is high, on the order of 50%. If pulses starting 

from zero bias are used, it is definitely advantageous to use a pulse 

scheme such as Manchester coding, where the duty cycle is relatively 

constant, as that will stabilize the average temperature. 

The conclusions to be drawn from the above discussion are the 

following: 

(a) Keeping a constant current flowing through the laser and 

varying the temperature, we expect a gradual shift in wavelength, and after a 2 to 4OC change, a jump by a few 8 to 

a new wavelength. 

(b) Keeping the temperature constant and varying the dc current, we expect similar behavior due to self-heating. 

(c) Keeping the TE temperature constant and increasing the pulse 

duty cycle, we expect the laser cavity temperature to shift; 

the amount of shift and corresponding mode jumps depend on 

the particulars of heat flow out of the laser. 

In the next section we describe experimental results on a CDH 

laser, and we show that the wavelength shift, even with standard 

pulse-code modulation, can be less than 10 8, our design goal. 

D. EXPERIMENTAL RESULTS 

1. Temperature-Induced Wavelength Changes 

The laser used for these tests was a CDH type with single-spectral 

line emission over a broad current and output power range. This laser 

12 
also had a single-spot near-field output with no discernible beam 

structure, and thus was typical of a good single-mode laser. 

(a) The laser was driven with lOO-ns pulses having an amplitude 

of 90 mA. The duty cycle was 0.01%. The laser was held on 

a TE and the heat-sink controlled at various temperatures 

while the spectrum was recorded. The results of the experiments are shown in Fig. 6. The stepwise increase in wavelength and the gradual shift are clearly seen. From this 

curve one obtains a value of -0.75 8/OC for the gradual, 

index-induced change in wavelength. To be sure, the slope 

from which this measurement is taken is somewhat different 

at different temperatures. The abrupt jump is about 5 8 

which is one longitudinal mode step for this laser. It is 

to be noticed that the change from one step to the next 

occurs in about 2OC. All of these results are in good agreement with the discussion given in section C. 

(b) The same laser, on the TE, was driven with a steady dc current of 91 mA. Here the results were similar although the 

position of the step was different. In the range of 16 to 

21°C there was no step (see Fig. 6) indicating that there 

exists a region in which the laser is more stable against 

mode shifts. In this case the total change was 3 8 over a 

temperature change of 5OC. Clearly this is a more desirable 

operating region for dc operation. In general, whether 

pulsed or not, one may expect that certain regions of operating temperature will provide a more stable wavelength in 

regard to mode jumps, and it is important to be able to set 

the laser at that temperature. A potentiometer on the TE 

power supply provides such an adjustment. 

(c) The laser was kept at a heat-sink temperature of 22.5OC while 

the pulse length was kept constant at 150 ns. The duty cycle 

13 
I5 

14 

13 

I2 

,F 

, - 

I I I I I I I I 

CDH SINGLE MODE .ti@ 

LASER, DB 68 r 

100 ns PULSE 

0.01% DUTY CYCLE 

90mA 

I I I 

15 I6 I7 I8 IS 20 21 22 23 24 

TEMPERATURE, “C 

Figure 6. Wavelength shift vs temperature. 

was varied from 1.5% where we assume no heating, to dc where 

we have the maximum heating. 

The results are given in Table 2 and shown in Fig. 7. 

T (“Cl 

22.5 

22.5 

22.5 

TABLE 2. EXPERIMENTAL RESULTS 

Duty Cycle (‘J,) Current (mA) Wavelength (8) 

1.5 77 8476 

15 77 8478 

100 77 8482.4 

14 
oa 

i 

6 8490 

E 

d 

$ 8480 

150 ns PULSE,77mA,22.5OC 

84700’ 

20 40 50 

DUTY CYCLE, % 

80 100 

Figure 7. Wavelength dependence on duty cycle, CDH laser. 

From this we obtain 0.083 x/mA for the gradual wavelength change 

with current where we assumed no further wavelength reduction in going 

from 1.5% to 0% duty cycle. Combining this with the previously 

established value for the temperature-induced wavelength change of 

0.75 8/OC we obtain O.ll°C/mA for the temperature change caused by internal heating. These figures are important for the present application because they show the maximum allowed current change for a given 

temperature and wavelength change. However, we also see that changing 

the duty cycle from essentially zero to 100% does not have to cause a 

mode jump even though the current-induced temperature change is large 

enough to produce mode jumping under some conditions. At some other 

operating temperature this stability may not occur, as shown in Fig. 6. 

The dashed line in Fig. 7 shows that the duty cycle dependence is far 

from linear. This departure is due to the fact that at low duty cycles 

the diode temperature rises linearly because the temperature depends 

15 
on adiabatic heating of a given volume'of material, whereas at longer 

duty cycles, the temperature depends on the mechanism of heat flow out 

of the diode. The wavelength change as the duty cycle is changed 

simulates the conditions in a pulse-modulation scheme where the information is coded as a variable pulse repetition rate. The total wavelength change with maximum measured duty cycle change is 6.4 8 which 

is below the required 10-8 maximum excursion. Even though the above 

data show that it is possible to operate the laser under these conditions, it seems more desirable to use a constant-input-power method 

such as Manchester coding. It may be mentioned that another method 

has been proposed to stabilize the wavelength [5], namely, one which 

supplies a current to the TE cooler whose magnitude is proportional to 

the current driving the laser. However, it seems that such a method 

succeeds only if the TE response time is fast enough to follow the 

temperature fluctuations, which does not seem to be an easy requirement. 

The present measurements can be used to obtain a value for the 

thermal resistance Rth for this laser. Rth is defined as the temperature increase resulting from a power input of 1 W expressed in "C/W. 

As we have seen, the temperature rise per unit current is O.ll°C/mA, 

and for an applied bias of 1.7 V we obtain 

O.llOC 

1.7 x 1o-3 

= 64OC/W 

Lasers with lower values of Rth would show smaller wavelength changes 

as the current or duty cycle are changed. 

5. M. Yamada, F. Iida, S. Kido, and R. Ishibashi, Trans. ICE Japan 

E6J, 896 (1978). 

16 
2. Output Fluctuations Due to Temperature Cycling 

It was observed that the output from the fiber end fluctuated with 

time even when the laser was driven with a dc signal. In order to elucidate the origin of this fluctuation, we performed several experiments. 

The current through the TE was displayed on an oscilloscope together 

with the signal picked up from the end of the fiber, using a fast 

optical detector. As shown in Fig. 8, the optical detector showed 

spikes synchronized with the TE driving pulses. These are, however, 

low in amplitude and are as fast as the current pulse. Thus it seems 

likely that they represent an electrical interaction in the circuit, 

such as a drop in the driver output due to the insufficient capacity 

in the 5-V supply or inadequate wiring. More precise data were obtained by measuring the output of the fiber with a calibrated detectordc amplifier system. We also recorded the temperature of the MCB using 

a sensitive thermocouple. In Fig. 9, curve c shows that the temperature excursions of the MCB due to the cycling of the pass transistor 

are 0.018OC peak to peak. 

a. CURRENT 

THROUGH TE 

b. OUTPUT FROM FIBER 

Figure 8. Output spikes due to TE pulsing. 

17 
POWER 

PCYNER 

TEMPERATURE 

(a) 

(b) 

cc> 

Figure 9. Comparison of fiber output with temperature 

(a) Laser driven with separate power supply 

driven with internal IC, and (c) temperatur 

,“f 

e 0 

the MCB. 

b) laser 

f MCB. 

Figure 9(b) shows the output from the fiber, and Fig. 9(a) the 

same output when the driving IC was bypassed and the laser operated 

from an external dc source. It can be seen that the periodicity in 

all three traces is the same, and thus attributable to the temperature cycling. That the fluctuation in Fig. 9(b) is due to electrical 

pickup is confirmed by the fact that it is eliminated when a separate 

power supply is used. The cycling visible in Fig. 9(a) is due to the 

18 
temperature-induced changes in threshold current. This can be seen 

by referring to Fig. 10 which shows the output for this particular 

laser measured at the fiber, for two different temperatures. From 

Fig. 10 it is established that a l°C change in temperature causes a 

*200-PW change in output. 

# 799 

5.0 - 

3 

E 

2.5 - 

40 60 80 

mA 

Figure 10. Laser output (measured at fiber end) for 19 and 25'C. 

For the measured peak-to-peak temperature excursion shown in 

Fig. 9(c) of 0.018OC we thus deduce an output change of 3.6 pW. The 

output change actually measured, as shown in Fig. 9(a), is 1.5 to 2 I.IW 

peak-to-peak; the discrepancy we believe to be due to the fact that the 

temperature shown in Fig. 9(a) was measured near the TE, and these pulsations are smoothed out at the laser position, some distance away from 

the TE. 

19 
The slower random fluctuations seen in Fig. 9 are probably caused 

by drift in the unstabilized dc amplifiers used in these experiments. 

The fluctuations shown in Fig. 9(a) amount to 0.05% of the peak output 

of 3.89 mW measured in this case. An electrical interaction between 

the two parts of the circuit, the driver and the TE supply, can be 

eliminated by making sure the wiring is adequate, by using a power 

supply of sufficient capacity, or by electrically decoupling the two 

circuits.'? The temperature control circuit is therefore considered 

to be satisfactory for the present application. 

3. Optical Feedback Effects 

It has been known for some time that reflection of radiation back 

into the laser leads to various undesirable effects [6,7]. In particular, the wavelength of the laser is affected by this feedback and 

there can be changes in modal properties. Other effects noted are 

the occurrence of spontaneous oscillations, threshold changes, noise, 

and instability, but we are mainly concerned with the effect feedback 

has on wavelength. An example of such a situation is shown in Fig. 11 

where we see spectra taken from the laser measured directly (without 

fiber) and after coupling to various fibers provided with hemispherical 

or flattened ends. Figure 11(a) shows the spectrum obtained when no 

fiber was used, Fig. 11(b), when the laser was coupled to a fiber 

having the end rounded very slightly, as shown in the photomicrograph. 

In this case, reflection from the fiber end is sufficient to disturb 

*NASA Langley has advised us that they have solved this problem by using 

remote sensing to control the power supply voltage. 

6. I. Ikushima and M. Maeda, IEEE J. Quant. Electron. QE-14, 331 

(1978). 

7. R. Lang and K. Kobayashi, IEEE J. Quant. Electron. QE-16, 347 

(1980). 

20 
I L 

I 

828Oii 8270 A 

(a) 

8360 % 

cc> 

A826Oii 

(b) 

Al 

8250% 

8260; 82iO i 8240; 

Cd) 

Figure 11. Optical feedback effects: (a) is output from laser 

before coupling; (b) after coupling to graded index 

fiber having the input end beaded as in photomicrograph; 

(c) same as (b) except that the end of the fiber (away 

from laser) was immersed in high index fluid; (d) output 

from fiber with more rounded input end. 

21 
the mode structure. When the fiber was removed, the direct output spectrum (not shown) reproduced that of Fig. 11(a). However, changing the 

reflection from the far end of the fiber pigtail by immersing it into a 

high-index liquid also produced a change [Fig. 11(c)], which indicates 

that undesirable feedback occurs from both the front and the back end 

of the fiber. Using a different, more rounded fiber end [Fig. 11(d)] 

again produced a single-mode spectrum, although at a different wavelength. These results suggest that coupling to a well-rounded fiber 

end can result in a single-line spectrum if the laser is initially 

capable of it, but that there is some optical feedback involved, from 

either the front or the back surface of the fiber, that affects the 

exact wavelength delivered. It seems likely that for best performance, 

a nonreciprocal element may have to be included in the optical path. 

The beneficial effect of the rounded hemispherical fiber end (beyond 

the increased coupling efficiency) is probably due to the reduced back 

reflection from the divergent optical system formed by the front surface of the lens. 

22 
II. COUPLING OF SINGLE-MODE LASERS TO 

THIN-FILM OPTICAL WAVEGUIDES 

A. INTRODUCTION 

The initial objective of this part of the program was to develop 

a method of coupling one of the longitudinal modes of a multimode junction laser to a diffused optical waveguide while not coupling or only 

weakly coupling the other longitudinal modes. 

Because of the inherent sensitivity of gratings to wavelength we 

investigated the properties of grating couplers to thin-film optical 

waveguides. This work is described in subsection B, below. 

We found that grating couplers do indeed have the property of 

coupling one wavelength more strongly than another and this property 

could be enhanced by increasing the coupling length. The strength of 

the coupling, however, depends inversely on the coupling length through 

undesired random scattering. Thus, while it was found possible to 

suppress an undesired axial mode spaced 6 8 from a desired mode by 

more than 20 dB, the overall coupling efficiency was less than 1%. 

Although improvement of the gratings to give higher coupling without 

sacrificing selectivity appears possible, a more practical alternative 

presented itself in that, as described in subsection B below, relatively 

high-power, single-longitudinal-mode junction lasers are now available 

which make mode selection unneccessary. 

Under these conditions it seemed advantageous to look at nonwavelength selective but more efficient coupling methods than the 

grating coupler. We thus decided to study prism couplers which are 

well understood and promised to provide compact and efficient couplers 

for transferring the single-longitudinal-mode of advanced junction 

lasers to optical waveguides. This work in turn was divided into two 

parts. 

23 
The first part is described in subsection C, below. Here we 

describe the development of a miniature cylindrical lens system which 

enables us to make a compact junction laser thin-film optical waveguide 

coupler. The lens system mounts directly on the laser and is small 

enough to be encapsulated with the laser in a header or other miniature 

protective seal. The lens system is designed to produce a symmetric 

beam which may, as a matter of choice, be either converging or diverging. 

At the same time, a large fraction of the laser light (greater than 90%) 

is collected. 

The experimental system described in subsection C brings the laser 

beam to a symmetrical focus with a beam waist of about 100 pm approximately 1 cm from the laser. The beam waist is chosen to be suitable 

for prism coupling to the optical thin-film waveguides used in this 

program. 

We think that the miniature cylindrical lens system described 

here will find many applications in conjunction with the use of junction lasers in fiber optics, thin-film optical waveguides, and data 

processing and storage systems such as optical disc recording systems. 

The actual coupling of the laser light to LiNbxTalDx03 (LNT) on 

LiTa03 waveguides and the construction of a small unit combining the 

laser cylindrical lens assembly with the prism-optical waveguide 

assembly which was delivered to NASA is described in subsection D. 

Single-wavelength power of over 0.5 mW in the optical waveguide was 

readily obtained. The coupling efficiency is approximately 10%. 

Much higher efficiencies are expected for future systems based on 

this approach. The demonstration reported here used less than optimum 

cylindrical lenses and mechanical arrangements to meet the time constraints of this program. 

We expect that optimized arrangements could demonstrate powers on 

the order of 2 mW in the waveguide. These arrangements do not require 

24 
any new technology or mechanisms, and the theory and art described in 

this report appear adequate to reach this latter value. 

B. SELECTIVE GRATING COUPLING 

Grating coupling to thin-film optical waveguides has been extensively described in the literature. We will briefly review the coupling 

conditions and then describe how the grating coupler may be used to 

preferentially couple a single wavelength mode from a group of axial 

laser modes. 

A schematic diagram of a thin-film waveguide and grating for 

coupling is shown in Fig. 12. We consider that a parallel beam of 

light of wavelength ho impinges on the grating at an angle to the 

waveguide normal of 8. In a focused beam the beam waist associated 

with the focused beam spot may be considered parallel for this purpose. 

The waveguide mode to be excited has an effective index ne. A phase 

grating is produced as an embossment in the waveguide as shown or, more 

commonly, as an added transparent corrugated layer placed on the waveguide surface. The grating period is d. In either case it is assumed 

that the grating does not appreciably perturb the waveguide properties. 

Under these assumptions the condition for coupling between the input 

beam and the waveguide is 

sin f3 =n 

0 e - Aold 

The intensity of light coupled will depend on the grating strength and 

the interaction length L. It must be noted that, as in the prism 

coupler described below, the grating couples light out of the waveguide 

as well as into the waveguide so that the grating must be terminated at 

some point within the input beam waist (W,). A complete discussion of 

this matter is given by Tamir [8]. 

8. T. Tamir, "Beam and Waveguide Couplers," in Integrated Optics, 

2nd ed., T. Tamir, Ed. (Springer-Verlag, NY, 1979) pp. 90-93. 

25 
WAVEGUIDE 

~b&-f-Dj SUBSTRATE 

Figure 12. Thin-film waveguide and corrugated grating coupler. 

The amplitude of the grating is exaggerated. 

If at the wavelength ho the angle go exactly satisfies Eq. (6), 

at some other value of wavelength A Eq. (6) will be exactly satisfied 

at a different angle (8). It has been shown that at a fixed wavelength 

the intensity coupled will vary with angle as 

where A8= f3 -9 I 0 I 

and I o is the intensity coupled at O. which exactly satisfied Eq. (6). 

If 0 remains constant and A is varied, we have 

(7) 

1M AU = - -- 

cos 8 d 

26 
giving 

where 

M= A-A 

0 

sin ( d:c;s;) 

I-ILM 

d2 COS 6 . 1 

2 

(8a) 

This may be written in a normalized form as 

I 

- = 

IO 

(8b) 

Equation (8) can be used to determine the coupling length required to 

reduce the coupling for an undesired axial mode at A spaced from a 

desired mode at A0 by AA = A - h . 

0 

The argument in Eq. (8) [II (L/ho) 

(Mlho) I/ [ WAo12 cos 81 may be written as 

Al (L/A01 (WAo) (9) 

where Al = U/(d/ho)2 COS 8 

Generally Al will be on the order of 10. Using Al (L/ho) as a parameter, we have plotted 10 Log (I/IO) or I/IO in dB as a function of 

M/A0 in Fig. 13. The dashed lines are the complete expression given 

by Eq. (8b) for a value of Al (L/ho) = 104. The solid lines connect 

the loss minima of Eq. (8b) and thus serve as boundaries for the lowest 

discrimination that will be obtained for given values of A1 (L/ho). 

The selectivity of the coupler depends on the coupling length. 

This is analogous to the use of gratings in spectrographic applications 

27 
i I i II ;’ ‘I 

i 

ll 

ll 

I’ II 

II 9 

II 

‘I 

!I 

II ‘I - 

II I! - 

E i 

II 

II Ii r; ii - 

II - 

I 

‘I I’ 

‘I I 1 I’ 

!I IL 

I l :I 

I : 

‘I - - 

Figure 13. Selectivity of waveguide grating coupler. The grating 

space and angle are chosen to optimize the coupling 

at A 0’ IO is the intensity coupled at ho. 1. the 

intensity coupled at some other wavelength A. 

where the resolution of the spectrograph is proportional to the grating 

aperture. In the coupler application it is desirable to have a large 

coupling length to suppress undesired modes. On the other hand, efficient coupling makes other demands on the coupling length. In theory 

efficient coupling would be independent of the coupling strength, requiring only that the coupling length be properly chosen to match the 

coupling strength. In practice, however, gratings with weak coupling 

strength requiring long coupling lengths suffer from the effect of 

imperfections much more severely than strong gratings requiring short 

coupling lengths. Thus, the actual requirement of high selectivity 

imposes limitations on the actual coupling percentage that will be 

practically available. 

28 
Experiment_ 

Optical Waveguide and Grating Preparation 

We used a LiNbxTalmx 0 3 on LiTa03 (LNT) optical waveguide produced 

prior to the start of this program. These guides are described by 

Hammer and Phillips [9,10]. The particular waveguide (LNT35155) was 

formed on a Y plate of LiTa03 that was pretreated with Li2C03. We 

deposited 800 8 of Nb on this plate and it was diffused at 1183OC for 

2+ hours in an O2 atmosphere. The resulting guide had a single TE and 

a single TM mode. The effective guide index (n,) was approximately 

2.18. 

The grating was formed by depositing a stripe of Shipley* AZ-1350 

photoresist 3 mm wide by 5 mm long. The long dimension was perpendicular to the "C" crystallographic axis. The photoresist was exposed 

in a holographic setup and developed to produce a grating with grating 

vector parallel to the "C" axis and grating space (d) of 0.66 pm giving 

a predicted coupling angle of 65O (measured 8 = 66O). 

Junction Laser Preparation 

RCA Laboratories AlGaAs laser sample 541-992 was fitted with two 

fibers mounted at right angles to each other in an attempt to obtain 

a symmetric beam. See subsection C for a complete description of the 

symmetrizing of junction laser beams using cylindrical rods as lenses. 

The work described in this subsection was done before the development 

described in subsection B. Here a 50-pm (0.002-in.) fiber was mounted 

close to and parallel to the junction plane and a second fiber of 380-ym 

diameter (0.015-in.) was placed in contact with the first fiber and at 

9. J. M. Hammer and W. Phillips, Appl. Phys. Lett. 24, 545 (1974). 

10. W. Phillips and J. M. Hammer, J. Electronic Materials 5, 549 (1975). 

*Shipley G. Inc., Newton, MA. 

29 
right angles to the junction plane. Both fibers were cemented in place 

with epoxy. The emerging beam was not quite symmetric but was a great 

improvement over the raw laser beam in angular symmetry. The diverging 

angle was approximately 3 degrees. A 20-cm focal length conventional 

lens was then used to focus the light on the grating. The distances 

were chosen so that the beam waist focused on the grating was approximately 1 mm (lo3 pm) in diameter. All the light coupled into the 

waveguide by the grating was extracted approximately 2 mm downstream 

from the grating edge by a prism film output coupler. The measured 

coupling efficiency was 0.6%. 

Light directly from the laser and light that had been coupled 

into the waveguide via the grating and then prism coupled out of the . 

waveguide was examined spectrographically. The resulting spectrographic traces are shown in Fig. 14. The results of careful measurements of the amplitudes are given in Table 3. The intensity of the 

8472-g line which is spaced 6 2 from the 8478-a line is reduced by 

the coupler such that 1(8472)/1(8478) = l-45/0.012 or a factor of 121. 

This corresponds to -20.8 dB. 

TABLE 3. MEASUREMENT RESULTS 

Wavelength (2) 

Relative Intensity 

Directly from Laser Through Grating Coupler 

and Waveguide 

8478 1.00 1.00 

8472 1.45 0.012 

8447 0.88 <O.OOl 

8421 0.43 <O.OOl 

8415 0.13 <O.OOl 

Other lines of relative intensity below 0.001 are not listed. 

30 
e47ei 

c 

SELECTIVE GRATING 

04721i 

COUPLING LASER CURRENT 89mA 

INTENSITY 

L WAVELENGTH 

THRU GRATING. W.G. AND OUTPUT 

8421; 

DIRECTLY FROM 

LASER 

Figure 14. Traces of the spectral output. Lower trace: 

directly from the junction laser; upper trace: 

light grating coupling into LNT waveguide and 

prism coupled out of waveguide. 

We may compare this with the theoretical expected results using 

Fig. 13. For our coupling angle and grating space the factor Al is 

calculated to be 12.85. With a l-mm spot (L = 1 mm) Al L/A = 15,297. 

Thus the curve for Al L/A = 15,000 will be approximately correct. 

For a 6-a spacing between lines we have M/A = 0.00071. With these 

values we would expect 18472/18478 to be -20 dB or less which is in 

good agreement with our observations. As expected, the other (further 

spaced) lines are reduced below detection sensitivity. 

We note again the difficulty of having an interaction length as 

great as 1,000 pm and still maintaining efficient coupling. The observed 0.6% coupling efficiency might be improved by better grating 

preparation. A very smooth grating with nearly perfect grating shape 

31 
would be required for any significant improvement in coupling efficiency 

without sacrificing resolution. With the availability of single-mode 

lasers it seemed more fruitful to combine a single-mode laser with other 

non-selective but more efficient coupling methods. 

C. MINIATURE CYLINDRICAL LENS SYSTEM FOR COLLECTING, SYMMETRIZING, 

ANU FOCUSING J-UNCTION LASER BEAMS 

1. Introduction 

In many applications of junction lasers, it would be desirable to 

render the highly divergent and axially asymmetric laser beam into a 

more tractable form. At the same time, whatever the application, it 

is necessary to use relatively high numerical aperture optics to 

collect all the laser light. Angular beam divergences at right 

angles to the laser junction plane of approximately 40° are common. 

In nonfiber coupling applications high numerical aperture microscope 

objectives or similar lenses have been used to collect all the light. 

When the application requires a symmetric beam, correcting cylindrical 

optics are used after the collecting lens. This arrangement frequently 

requires very long total optical path lengths (-1 meter), creating 

very bulky systems. 

In this subsection a relatively simple collecting and symmetrizing 

cylinder lens system will be described. In this system, illustrated 

in a general form in Fig. 15, two small cylinders of refractive indexes 

and diameters chosen on the basis of the theory below are set with axes 

perpendicular to each other. The cylinder nearest the laser is arranged 

with its axis parallel to the junction plane. The cylinders collect 

the laser light and result in an emerging beam which retains a "square" 

cross section as it propagates away from the lens system. In many 

applications the lenses can be chosen so that no further optical elements are required - i.e. , prism or grating coupling to planar or 

32 
Figure 15. Perspective illustration of cylinder lens system 

for symmetrizing junction laser output. 

stripe optical waveguides. (See subsection D.) The lenses are located 

sufficently close to the laser and are of sufficently small size that 

the laser and the lenses may be considered a unit that might readily 

be encapsulated or hermetically sealed. 

2. Theory 

The calculation of the required lens diameters, refractive indexes, 

and positions is relatively straightforward. In Fig. 16 the rays at 

the half-power angles in both transverse and lateral direction and the 

lens cross sections are superimposed in one plane so that the required 

trajectories become apparent. For this figure we imagine the lateral 

direction rotated around the Z axis of Fig. 15 by 90°. In Fig. 16, 

the rays and lens cross section shown by solid lines lie in the Y-Z 

33 
Figure 16. Ray diagram for calculating cylinder lens parameters to 

symmetrize junction laser output. The lateral direction 

is shown by dashed lines and has been rotated 90° around 

the Z axis. 

plane and are referred to as Transverse quantities using the subscript 

T. This Y-Z plane is perpendicular to the junction plane of the laser. 

The rays and lens cross section shown using dashed lines lie in the X-Z 

plane which is parallel to the junction plane and are referred to as 

Lateral quantities using the subscript L. 

We consider the case where both lenses are right circular cylinders. 

20 is the transverse angular spread of the laser measured at the halfpower point; 21$ is the lateral angular spread of the laser similarly 

measured. 201 is the desired angle of convergence after the lens 

system. Clearly, to meet requirements of symmetrizing the beam, el 

must be the same in both the lateral and transverse planes. 

We assume that in the transverse direction the beam originates 

from a point source located at the laser facet. The point is actually 

the intersection of a line parallel to the X axis (in the junction 

plane) and the Y-Z plane. The length of this line is the width of the 

lateral laser mode (1,) at the laser facet. The width of the transverse mode (~1.0 pm) can be ignored with negligible error in this lens 

34 
calculation. In the lateral plane we assume the beam originates at a 

point source a distance aL < !J /2 

1 

_ L tan $I behind the laser facet. 

aLl 

depends, in fact, on the character of the beam waist at the laser facet. 

For some lasers a 

Ll 

s 0. For a complete circular cylinder the principal 

plane degenerates to a single plane through the cylindrical axis. aT, 

(a -a L Ll ) are the physical distances of the transverse and lateral lens 

axes from the light sources. bT, bL are the respective distances to the 

beam waist W. W is required to be equal in both the transverse and 

lateral directions. Because of the presence of the transverse lens 

in the object space of the lateral lens, a L is less than the optical 

length of the lateral object distance. Similarly, the presence of the 

lateral lens in the image space of the transverse lens makes bT less 

.than the optical length of the transverse image distance. 

We compensate for this in the subsequent calculation by increasing 

the lateral object distance by the first order correction (n,-1) DT 

and the transverse image distance by (nL-l)DL. Here, and in the 

remaining calculation, n T is the refractive index of the transverse 

lens and nL that of the lateral lens. 

The focal length of a circular cylinder of refractive index n and 

diameter D is f = (4 (n-l)2/n)D [ll]. Thus, 

fT = 4 (nT-1)2/nT 1 DT 

fL = 4 (nL-1)2/nL 1 DL 

(10) 

11. M. Born and E. Wolf, Principle of Optics, 2nd Ed. (Macmillan, NY, 

1964) p. 161. 

35 
The lens formulas can be written as follows: 

1 1 1 

+ afy 

.- 

+ bL 

= - 

aL fL 

1 1 1 - - 

aT + bT + bfL = fT 

To ensure that CIl and W are equal for both planes requires that 

( aL + af T 

> 

tan Cp aT tan 8 

= 

bL bT + bf L 

The value of Cl1 is given by 

tan Cl1 = 

aT 

bT + bfL tan 8 

(11) 

(12) 

(13) 

(14) 

Here the appropriate optical path lengths are corrected for computational 

convenience in the form af T and bf L where 

4 (nT-1)3 

a= 

nT 

(15) 

b= 

4 (nL-1)3 

nL 

These are the first order corrections mentioned earlier. Finally, to 

ensure that the beam waist lies at a common point along the Z axis for 

both the lateral and transverse plane, it is necessary that the physical distance of the beam waist from the laser facet be the same in 

both planes. Thus, 

aT + bT = aL - aLl + bL (16) 

36 
We know 8, I$, and aL and we choose D1 or W which are not independent. 

1 

We also choose nT, nL and D T or ft which leaves the five unknowns; DL 

related to f L through Eq. (lo), aT, bT, aL and bL. We have the five 

equations relating these quantities: Eqs. (ll), (12), (13), (14), and 

(16). 

In addition we have 

A 

tan D1 = $ 

which relates the beam waist size to 8 1' 

"convergence" ratio 

tan e1 A 

B=- = 0 

tan 8 WT tan 8 

(17) 

If we define a transverse lens 

(18) 

and define the laser transverse to lateral divergence ratio 

A _ tan 8 

-- 

tan $ 

We find the following solution 

fL = 

(BA + 1) 

BA 

' 2 + b] ifT [$$ + ‘1 + ‘L1i 

aT = 

fT (B + 1) 

bT = fT (; + 1) - bfL 

aL = fL (BA + 1) - afT 

1 bL = fL (sa + 1) 

We repeat Eq. (10) 

fT = 14bT - 1)2/nT] DT ; fL = [4(nL - 1)2/nLl DL 

(19) 

(20) 

(21) 

(22) 

(23) 

(24) 

37 

I- - 
For the solutions of Eqs. (20) th rough (24) to be physically realizable, 

the distance G between the lens surfaces must be greater than or equal 

to zero. G is given by 

G = (aL - aL 

1 

- DLL21 - caT + DT/2) (25) 

In addition it is desirable to have both DT and DL large enough to 

accept all of the beam. The maximum angles accepted are given by 

eM = sin -' DT/2aT and aM = sin -' DL/2aL (26) 

in the transverse and lateral directions, respectively. The rejection 

ratios G/G, and $I/@, should be as small as possible to ensure maximum 

beam acceptance, minimum cylindrical distortion and make as close an 

approximation as possible to the first-order length corrections. When 

G/G, > 1 or I$/$~ > 1, all the light is not collected. 

To give some appreciation of the implications of these equations 

we have plotted some of the results for a particular case. We assume 

a junction laser with a transverse beam spread of 28 = 40° and a lateral 

beam spread 21$ = 8O. These values are high but still typical of many 

GaAs lasers. 

Equations (17) through (26) and (10) are used as follows: nL, 

nT' and D T are chosen. Then for a desired beam waist W or convergence 

angle 8 (note from Eq. (17) that these are not independent), the 

required lateral lens diameter DL and the spacings are calculated. 

All of the spacings and the lateral lens diameter can be determined 

as a fraction or multiple of the chosen transverse lens diameter. 

Thus, in Fig. 17, we plot the "normalized" lateral lens diameter 

DL/DT and the "normalized" spacing of the transverse lens from the 

laser facet aT/DT as a function of normalized beam waist W/A0 for nT 

values of 1.5, 1.6, 1.7, and 1.8. The lateral lens is assumed to 

have a refractive index of 1.5 (Pyrex, for example). For the same 

38 
values of refractive indexes the normalized distance between the 

lenses G/DT and the rejection ratio of the transverse lens O/G, are 

plotted as a function of W/ho in Fig. 18. Here note that for nT = 

1.8 the required distance between lenses G/DT becomes less than zero 

for W/A0 less than 2. Thus, it is not possible to achieve this condition. Also note that the rejection ratio is greater than one (not all 

the light is collected) for values of W/A0 less than about 3. Thus, 

for the particular laser considered (28 = 40° and 2@ = 8O) it is not 

possible to directly obtain spot diameters below about 3 A0 while also 

collecting all the laser light using only the cylinder lens system. 

What happens for very small beam waists is that if the refractive 

index is increased in an attempt to collect all the light, the lenses 

overlap. If the refractive index is decreased to avoid the overlap, the 

lens optical aperture is reduced below that required for full collection. 

Figure 17. Plot of DL/DT and AL/DT as a function of normalized 

beam waist W/A 

0’ 

The lens output "convergence" angle 

Gl is shown on the upper scale. The refractive index 

of the transverse lens n T is the parameter. 

39 
ai t 

Figure 18. Plot of G/DT and 0/O, vs W/A 

0 

; nT is the parameter. 

When G < 0, the lenses overlap. When O/G, > 1, not all 

of the laser light is collected at n = 1.8 G/DT < 0 

(negative) when W/A0 2. 2.0. 

At larger beam waists and smaller convergence angles this problem is 

avoided. 

It should also be noted that Gl can be chosen to be negative 

(diverging symmetric beam) for some applications. Here there will 

be a virtual beam waist on the object side of the lenses. 

The plots of Figs. 17 and 18 are included to illustrate the calculations; for any given laser the formulas presented are in closed 

form and the required values may be directly calculated as outlined 

earlier. 

D. PRISM COUPLING TO OPTICAL WAVEGUIDES 

The prism film coupler to thin-film optical waveguides has been 

described in great detail in a number of publications (ref. 8, pp. 86-90, 

40 
101-110). We will thus only summarize the important parameters involved 

in this coupling method. 

Referring to Fig. 19, a laser beam is focused to have a beam waist 

W measured in the plane perpendicular to the waveguide. The beam waist 

in the plane parallel to the waveguide is less important; however, it 

must be considered. A large beam waist in the parallel direction raises 

the problem of skewness with the edge of the prism. We will thus consider that whatever the parallel beam waist dimension, it indeed is 

kept parallel to the waveguide plane and the prism edge. This problem 

is simplified for symmetric beam waists. With this caveat in mind we 

proceed with our description. 

Restricting our attention to the beam waist perpendicular to the 

waveguide, we note that the beam waist center is offset from the corner 

formed between the prism base and perpendicular prism face by a distance Z less than W. The prism base is separated from the waveguide 

plane by a gap g which is less than a wavelength of light. 

Light entering the prism will undergo total internal reflection 

from the base. The gap and angles are adjusted so that the evanescent 

WAVEGUIDE 

SUBSTRATE 

Figure 19. Schematic diagram of a waveguide-prism film coupler. 

41 
field of the totally internal reflected light in the prism and that of 

a waveguide mode that would be excited overlap in the gap region and 

have the same phase velocity parallel to the waveguide plane. Under 

these conditions energy will be coupled between the waveguide mode and 

the incident light. Initially all the light is in the prism; hence, 

light is lost from the prism to the waveguide mode. As the mode intensity builds up, light will be coupled from the waveguide back to 

the prism. The interaction must then be stopped at a point where the 

waveguide mode has been excited but has not yet lost too much light 

back to the prism. This is accomplished by having the beam waist, 

the gap, and the offset properly chosen (ref. 8, pp. 86-90). 

The required phasematch condition is 

n /A -COS 8 = n PO e 

"P 

must be greater than ne for coupling. ne is the effective index of 

the guided mode in question, and n 

P 

is the index of the.prism. 

The required waist size depends on the strength of the coupling 

which in turn depends on the gap width. Narrow gaps produce stronger 

coupling coefficients and require small beam waists. The gap, in 

practice, is adjusted by varying the pressure used to press the prism 

onto the waveguide. We have found beam waists in the 50- to 200-pm 

range can be accommodated with the mechanical arrangements used in our 

Laboratories. The correct offset Z is obtained by adjustment during 

alignment. 

It has been shown (ref. 8, pp. 86-90) that uniform gap couplers 

can have maximum input coupling efficiency of approximately 80%. This 

value can be raised to 100% if a tapered gap coupler is used. In the 

output coupler, since light is continuously removed from the interaction 

region, 100% output coupling is achieved if the prism base is in close 

proximity to the waveguide for a sufficiently large distance. 

An important advantage of the prism-film coupler is the fact that 

neither the beam waist nor the offset dimensions are critical. For 

42 
example, in a uniform gap coupler changing the beam waist from its 

optimum value by a factor of two only reduces the predicted maximum 

coupling efficiency from 80% to 70%. 

We now calculate the required lens system to produce a beam waist 

of approximately 100 pm for use in a prism coupler to couple light from 

a junction laser to a LNT waveguide. We use the formulas given in 

subsection C.2 above. By symmetrizing the beam we will ensure that 

the parallel beam waist is sufficiently small to avoid coupling problems due to skew direction. Since we had available a sapphire cylinder of 0.0965-cm diameter, n =n T = 1.76, we made the calculation 

for using this cylinder as the transverse lens and a quartz cylinder 

with n = n L = 1.4526 as the lateral lens. Laser 732 which we used 

for the coupling has 8 = 18O and I$ = 3.25O. Using these values we 

obtain D L = 0.189 cm, aT = 0.0754 cm, bT = 2.866 cm, aL = 0.311 cm 

and b L = 2.630 cm, G = 0.083 cm and O/8, = 0.45. 

E. EXPERIMENTAL OBSERVATIONS AND DEVICE CONSTRUCTION 

A lens holder was designed similar to that shown in Fig. 20. In 

the original design, the groove depths are chosen to give the calculated 

value of the gap G between the lenses. The holder is positioned in 

front of the laser using a micromanipulator which provides x,y,z motion. 

Provision is made to allow rotation of the holder to ensure that the 

transverse lens is parallel to the junction. When a final lens alignment is made, as verified by measurements of the beam waist described 

below, the holder is cemented to the laser mount using epoxy. While 

the epoxy is setting+ the beam waist may be monitored to allow small 

adjustments to compensate for any shrinkage or other motion of the 

epoxy. 

As mentioned, the transverse lens was made from a stock sapphire 

rod. The rod was not accurately round, with variations on the order of 

43 
SLOT FOR LATERAL CYLINDER 

SLOT FOR 

LENS 

Figure 20. Sketch of an improved cylinder lens holder. 

The slots are spaced so that the gap is zero. 

Shims are used to give the desired gap and the 

cylinders are cemented into place. 

+ 0.001 cm. It was felt, however, that it would be adequate for verification of the technique. The lateral lens was fabricated by grinding 

and polishing a quartz rod. Because of some problems in the final 

polishing step, the actual lens used had a diameter of 0.193 cm rather 

than the 0.189 cm called for by the calculation. We expected and found 

that small variations of this order can be compensated for in the final 

lens adjustment although the final beam waist size would vary from the 

nominal value of 100 pm. 

The beam waist is measured using a slit detector assembly which is 

held on an x-y-z micromanipulator. The slit may be rotated to measure 

either the transverse or lateral beam waist at the same z position. 

The detector output is placed on the ordinate and the slit position on 

the abscissas of an x-y recorder plot. The minimum beam waist is found 

by scanning at different axial (z) positions. 

Scans of the lateral and transverse beam waists obtained after the 

lens system is cemented into position are shown as part of Fig. 21. The 

beam waists are centered at a distance of approximately 2.5 cm from the 
LENS SYSTEM OUTPUT 

TRANSVERSE BEAM WAIST 

(WT) 

LENS SYSTEM OUTPUT 

OPTICAL WAVEGUIDE OUTPUT, “FAR FIELD” SCAN 

Figure 21. Experimental x-y trace of the lateral and transverse 

beam waist. The center figure is on x-y tracing of 

the far-field pattern of light coupled out of the 

LNT guide. 

lens system. The depth of field is sufficiently long so that it is 

difficult to measure waist size differentials within about 0.5 cm from 

the waist center. 

The observation of the symmetry and approximately correct beam 

waist size and location give reasonable agreement with the theoretical 

design of the miniature lens system. 

The laser-lens assembly is then mounted in a compact assembly to 

bring the beam waist to the correct position at the coupling prism for 

prism coupling to an LNT waveguide. The same mount provides means of 

squeezing the prism in place on the LNT waveguide. 

Figure 22 is a schematic diagram of the relative position of 

various components in the laser-lens waveguide assembly. A crosssectional view of the mechanical details of the laser-lens waveguide 

mount is shown in Fig. 23. Photographs of the completed unit which 

45 
JUNCTION 

SMA CENTER LASER 

TERMINAL 

NEGATIVE \ 

CYLINDER LENSES 

TRANSVERSE ,097 cm DIA. SAPPHIRE 

LATERAL . I93 cm DIA. QUARTZ 

60° SrTiOS 

COUPLING PRISMS 

DE 

L LENS HOLDER 

CEMENTED TO I 

L-MOUNT 

Figure 22. Schematic of laser-cylinder lens LNT waveguide assembly. 

SPRING LOADED 

L-MOUNT HOLDER 

\ y LASER L-MOUNT 

BASE 

‘RESS ,PRISM PRESS ARM 

WAVE GUIDE PRESS 

TABLE 

PIVOT ARM 

Icm 

H 

Figure 23. Cross-section of mechanical module showing 

details of laser-lens-LNT waveguide mount. 

was shipped to NASA Langley are shown as Figs. 24 and 25. Optical 

power output vs laser current and the spectrum measured directly from 

the laser is shown in Fig. 26. The optical power and spectrum measured 

after the lens assembly is cemented in place are shown in Fig. 27. 

46 
Finally, the optical power output from the waveguide vs laser current and the spectrum are shown in Fig. 28. Here we are looking at 

the power which has been coupled into, transmitted through, and 

coupled out of the waveguide by the output prism coupler. The power 

represents a lower bound on the optical power flowing in the waveguide. 

The particular LNT sample has a loss of approximately 0.8 dB/cm. The 

interprism distance is 4 mm so that the insertion loss due to the waveguide is 0.4 x 0.8 dB = 0.32 dB, or a fractional transmission of 0.93. 

Even neglecting the waveguide loss, we see that approximately 10% of 

the laser light is coupled out of the waveguide in the miniature 

assembly. Thus a minimum of 0.5 mW of light is traveling in the 

waveguide at a laser current of 60 mA. 

We consider this an encouraging result. The mechanical arrangement is subject to much improvement and did not allow for the fine 

control required to fully optimize the coupling. The lenses used 

were not quite optimized, and in particular the sapphire lens was not 

of good optical quality. The lenses were not antireflection coated, 

which alone causes a nonessential 20% insertion loss (at 60-mA laser 

current , power reduced from 4.9 mW at the laser to 3.9 mW through the 

lens system). There is every reason to expect that under fully optimized conditions this type of compact arrangement will yield laser 

waveguide coupling fractions of greater than 50%. 

Finally as an illustration of the waveguide quality a far-field 

trace of the light coupled out of the waveguides taken with a moveable slit both parallel and normal to the waveguide plane is given in 

Fig. 21. The far-field trace normal to the plane shows a pattern 

typical of prism couplers. This has a relatively sharp rise in intensity near the prism edge. The fall-off is more gradual as more 

and more downstream light is removed as it passes further under the 

output prism. 

47 
Figure 24. Photograph of final laser-lens-waveguide module. 

48 
Figure 25. Photograph of laser-lens-waveguide module 

in protective plastic case. 

49 
0 20 40 60 

DC CURRENT (mA) 

--a 

3440 8430 6420 I I I I I I 

: SPECTRUM 

LASER CURRENT 

55mA 

LASER OUTPUT 

Figure 26. Plot of laser power vs laser drive current and 

spectrum of laser output. 

--a 

150 8440 0430 84; 

I I I I I 

SPECTRUM 

LASER CURRENT 

54mA 

LENS SYSTEM OUTPUT 

3 

LASER CURRENT (mA) 

Figure 27. Plot of light intensity output of lens system vs laser 

drive current and spectrum of lens system output. The 

input is that shown in Fig. 26. 

50 
, - 

Is- 

,- 

IL 

0 20 40 60 

LASER CURRENT (mA) 

-A 

440 8430 8420 841 I I I 

SPECTRUM 

LASER CURRENT 

54 mA 

OPTICAL WAVEGUIDE OUTPUT 

Figure 28. Plot of light intensity output coupled out of LNT 

guide of module vs laser current and spectrum of 

same. The input is that shown in Fig. 27. 

Parallel to the waveguide plane the coupling traces, to a reasonable extent, the input lateral beam waist which is also shown in the 

drawing. 

Over the range scanned the in-plane waveguide scattering is small. 

We estimate the in-plane scattering to be below 20 dB. 

We believe these experiments demonstrate the feasibility of using 

compact cylinder lenses and prism couplers to construct simple, compact, 

and efficient couplers between junction lasers and optical waveguides. 

Similar lenses can be used in other applications of junction lasers 

which will benefit from a symmetric and low divergence optical beam. 

51 
III. CONCLUSIONS 

A temperature-stabilized laser module has been developed which 

is suitable for providing constant wavelength output pulses needed 

for optical-fiber wavelength multiplexing systems. It consists of 

a thermoelectrically stabilized laser, a driver, and the required 

power and control circuits. The module requires a single 5-V input 

with a rating of 1.5 A and a TTL signal input, both entering through 

one cable connector. The output is taken from a fiber-bulkhead 

connector built into the module. The wavelength stability obtainable 

from laser sources is discussed in general and has been measured experimentally. It is shown that for CDH lasers operating under worst-case 

conditions (large duty cycle changes during non-return to zero coding) 

the wavelength shift can be held within 6.4 8. In other pulse coding 

schemes the shift is expected to be a great deal smaller. To obtain 

the best performance, it appears desirable to tune the laser temperature 

over a few degrees in order to find a temperature region which provides 

the most stable spectral output. This is easily accomplished in the 

present thermoelectrically controlled system. 

The return of part of the optical signal because of reflections 

at various interfaces has been found to affect the emitted wavelength. 

For best performance, a non-reciprocal device may have to be placed 

between the laser and the fiber. 

Basic principles of temperature-dependent behavior of injection 

lasers are discussed, as they should be useful in the design of future 

laser modules. 

In the second part of this program, coupling of a single-wavelength mode of a junction laser to in-diffused LNT (LiNbxTalmx03 on 

LiTa03) optical waveguides has been demonstrated. 

Using single-mode lasers, efficient prism coupling to LNT guides 

was observed. A miniature and widely useful cylinder lens system 

52 
has been developed for this purpose. The lenses are sufficiently 

small to be incorporated with the laser in a unitized mount which 

may be encapsulated or hermetically sealed in standard type headers. 

A compact single-mode-laser-miniature-cylinder lens-LNT-waveguideprism-coupler assembly was prepared and delivered to NASA. In this 

unit, which was not optimized because of time limitations, 0.5 mW 

of laser light was coupled into and out of the LNT guide at a laser 

current of only 60 mA (dc). By following the procedure described in 

detail in this report, it should be possible to construct miniature 

units of this type in which 2 mW of single wavelength junction laser 

light is coupled into thin-film optical waveguides. 

Earlier in the program we demonstrated that grating couplers 

can be constructed which selectively couple one of the wavelength 

modes of a multimode junction laser to an LNT waveguide. To obtain 

the required selectively, however, it is necessary to use a long 

grating where scattering due to grating imperfections makes the 

realization of efficient coupling difficult. To obtain both high 

selectivity and efficient coupling, improvements in the perfection 

of coupling gratings would be required. 

53 
REFERENCES 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

M. B. Panish and H. C. Casey, Jr., J. Appl. Phys. 40, 163 (1969). 

M. Yamada and Y. Suematsu, IEEE J. Quant. Electron. QE-15, 743 

(1979). 

M. Cardona, Int. Conf. Semicond. Phys., Prague, 1960, p. 388. 

S. I. Novikova, Sov. Phys. Solid State 3, 129 (1961). 

M. Yamada, F. Iida, S. Kido, and R. Ishibashi, Trans. ICE Japan 

E61, 896 (1978). 

I. Ikushima and M. Maeda, IEEE J. Quant. Electron. QE-14, 331 

(1978). 

R. Lang and K. Kobayashi, IEEE J. Quant. Electron. QE-16, 347 

(1980). 

T. Tamir, "Beam and Waveguide Couplers," in Integrated Optics, 

2nd ed., T. Tamir, Ed. (Springer-Verlag, NY, 1979) pp. 90-93. 

J. M. Hammer and W. Phillips, Appl. Phys. Lett. g, 545 (1974). 

W. Phillips and J. M. Hammer, J. Electronic Materials 4, 549 

(1975). 

M. Born and E. Wolf, Principles of Optics, 2nd Ed. (Macmillan, 

NY, 1964) pp. 161. 

54 
1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No. 

NASA CR-3341 

4. Title and Subtitle 5. Report Date 

SINGLE-MODE LASER STUDIES: (I) DESIGN AND PERFORMANCE October 1980 

OF A FIXED-WAVELENGTH SOURCE AND (II) COUPLING OF LASERS 6. Performing Organization Code 

TO THIN-FILM OPTICAL WAVEGUIDES 

7. Author(s) 

I. Ladany and J. M. Hammer 

9. Performing Organization Name and Address 

8. Performing Orgamzation Report No. 

PRRL-80-CR-13 

10. Work Unit No. 

RCA Laboratories 

Princeton, New Jersey 08540 

12. Sponsoring Agency Name and Address 

11. Contract or Grant No. 

NASl-15440 

13. T pe of Report and Period Covered tontractor Report 

(7-3-79 to 4-3-80) 

National Aeronautics and Space Administration 

Washington, DC 20546 

14. Sponsoring Agency Code 

15. Supplementary Notes (Final Report No. 2) 

Langley Technical Monitor: Herbert D. Hendricks 

Topical Report 

16. Abstract 

Iwo applications of single-mode lasers have been studied. In the first, a module was 

developed for the generation of a stable single wavelength to be used for a fiberoptic multiplexing scheme. The laser is driven with RZ pulses, and the temperature is 

stabilized thermoelectrically. The unit is capable of maintaining a fixed wavelength 

within about 6 A as the pulse duty cycle is changed between 0 and 100 percent. This 

is considered the most severe case, and much tighter tolerances are obtainable for 

constant input power coding schemes. Using a CDH laser, a wavelength shift of 0.083 

A/mA is obtained due to laser self-heating by a dc driving current. The thermoelectric unit is capable of maintaining a constant laser heat-sink temperature within 

0.02Oc. 

In the second part of the program miniature lenses and couplers have been developed 

which allow efficient coupling of single wavelength modes of junction lasers to thinfilm optical waveguides. The design of the miniature cylinder lenses and the prism 

coupling techniques described will allow 2 mW of single wavelength mode junction laser 

light to be coupled into thin-film waveguides using compact assemblies. Selective 

grating couplers were also studied. 

7. Key Words (Suggested by Author(s)) 

Single-mode lasers 

Fiber-optic multiplexing scheme 

18. Distribution Statement 

Unclassified - Unlimited 

19. Security Classif. (of this report) 

Unclassified 

I 

20. Security Classif. (of this page) 

Unclassified 

Subject Category 36 

21. No. of Pages 22. Price 

60 A04 

For sale by the National Technical information Service, Springfield, Virginia 22161 

NASA-Lang1 ey. 1980