Mode-locking is stabilized effectively by conventional loss-modulator and phase-modulator, mode-locking elements placed in laser cavity in optical series with one another. Resulting dually-mode-locked system provides pulses with constant phase relative to mode-lock drive signal without presence of relaxation oscillation noise

Osmundson, J.

Rowe, E.

Santarpia, D.

English

NASA Technical Reports Server

• July 1974	 B74-10038 
NASA TECH BRIEF 
•	 Goddard Space Flight Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating 10 the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Dually-Mode-Locked Nd:YAG Laser 
.
The problem: 
Mode-locked Nd:YAG lasers have been contem-
plated for use as transmitters in pulse-code-modulated 
(PCM), optical communication systems. In such systems, 
a modulator external to the laser transmitter either 
passes or blanks each mode-locked pulse according to a 
binary coding scheme, thus producing a stream of ones 
and zeros at an optical receiver. The problem with mode-
locking, however, is that the drive frequency which 
produces the internal time varying amplitude or phase 
perturbation of the laser must be matched exactly to the 
laser cavity length, in order to have good pulse ampli-
tude, repetition rate, and width stability. Since the 
laser cavity varies continuously in length due to the 
thermal expansion and contraction and acoustical effects, 
stabilization is achieved only by continuously compen-
sating for these effects. Different techniques have been 
used to stabilize mode-locking with limited effectiveness.
The solution: 
Mode-locking is stabilized effectively by conven-
tional loss-modulator and phase-modulator, mode-lock-
ing elements placed in the laser cavity in optical 
series with one another. The resulting dually-mode-
locked system provides pulses with constant phase 
relative to the mode-lock drive signal without the 
presence of relaxation oscillation noise. 
How it's done: 
Figures 1(a) and (b) show schematics of two laser 
cavity configurations utilizing the dually-mode-locked 
concept. In Figure 1(a) the phase mode-locker is 
adjacent to the rear mirror and the loss modulator is 
adjacent to the front mirror. In Figure 1(b) the phase 
mode-locker is adjacent to the rear mirror and the loss 
mode-locker is just to the right of the phase mode-locker. 
ri	 I  Laser Head Phase	 Nd:YAG Rod	 Loss 
Mode- __ I i
	 -----------i - —1 Mode-
Locker
	 I-JU 
Rear 
	
Front 
Mirror	 (a)	 Mirror 
-
 —I
Phase[	
-- 
Mode- -	 IVIode- 
[Locker	 Locker 
Rear 
Mirror
Laser Head 
Nd:YAG Rod 
-I	 -	 ---4---
(b)
--0 
Front 
Mirror 
Figure 1. Laser Cavity Configurations: (a) Dually-Mode-Locked Laser With a Loss 
Mode-Locker Adjacent to the Front Mirror and a Phase Mode-Locker 
Adjacent to the Rear Mirror, (b) Dually-Mode-Locked Laser With a Loss 
Mode-Locker in a Different Position
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19740000038 2020-03-17T14:00:58+00:00Z
Phase Mode-Locker 
Figure 2. An Optical Pulse Propagating From the Phase Mode-Locker to the Loss 
Mode-Locker 
There are two important considerations when imple-
menting a dual loss and phase mode-locker system for 
optimum performance. First, the two mode-lockers must 
be driven at different modulation frequencies in order 
that each produce an output pulse train of the same 
repetition rate. The second consideration is that mode-
locked pulses must be properly timed in order to pass 
through both mode-lockers without being extinguished. 
Figure 2 shows a mode-locked pulse leaving the phase 
mode-locker and propagating toward the loss mode-
locker. This pulse must arrive at the loss mode-locker 
when transmission through the loss modulator is at 
maximum. Timing can be accomplished by adjusting the 
phase of the modulation signal applied to one of the 
two mode-lockers; Figure 2 shows a simple case where 
both mode-lockers are driven by the same source, and 
the phase of the signal to the phase mode-locker is 
adjusted. 
In this system, the loss mode-locker is driven at a 
very low level so that the mode-locked pulses are 
extremely wide and there is no relaxation oscillation 
noise. When operated in this condition, the loss mode-
locker acts as a gate, forcing the phase mode-locker to 
remain always in phase with the mode-lock drive 
signal.
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Goddard Space Flight Center 
Code 207.1 
Greenbelt, Maryland 20771 
Reference: TSP74-10038 
Patent status: 
This invention is owned by NASA, and a patent 
application has been filed. Inquiries concerning non-
exclusive or exclusive license for its commercial develop-
ment should be addressed to: 
Patent Counsel 
Goddard Space Flight Center 
Code 204 
Greenbelt, Maryland 20771 
Source: John Osmundson, Edward Rowe,

and Dominick Santarpia

Goddard Space Flight Center

(GSC-1 1746) 
B74-10038	 Category 03,02
0


Dually-mode-locked ND: YAG laser

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