May 2011

NASA/TM–2011-217091

Amplitude Noise Reduction of Ion Lasers With 

Optical Feedback 

Gregory C. Herring 

Langley Research Center, Hampton, Virginia 
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May 2011 

NASA/TM–2011-217091 

Amplitude Noise Reduction of Ion Lasers With 

Optical Feedback 

Gregory C. Herring 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 
1

Contents

Abstract ............................................................................................. 2

Introduction ......................................................................................... 2

Experiments and Results ........................................................................... 2

Summary ............................................................................................. 4

References ........................................................................................... 5

Figures ................................................................................................ 7

List of Figures

Figure 1 Schematic of the setup used for observing amplitude and frequency 

characteristics of the laser output when providing optical feedback into an

Ar-ion laser cavity.

Figure 2 Frequency spectrum of Ar-ion laser (a) without feedback showing so 

many lasing modes that the spectrum appears as hash and (b) with optical 

feedback from a BaTiO3 PCM showing a greatly reduced number of 

modes.

Figure 3 Time trace of Ar-ion laser output (a) without feedback showing 85 MHz 

oscillations due to mode beating between adjacent longitudinal modes and 

(b) with feedback from a conventional mirror showing significantly 

reduced oscillations.
2

Abstract

A reduction in amplitude noise on the output of a multi-mode continuous-wave 

Ar-ion laser was previously demonstrated when a fraction of the output power was

retroreflected back into the laser cavity. This result was reproduced in the present work

and a Fabry-Perot etalon was used to monitor the longitudinal mode structure of the 

laser. A decrease in the number of operating longitudinal cavity modes was observed 

simultaneously with the introduction of the optical feedback and the onset of the 

amplitude noise reduction. The noise reduction is a result of a reduced number of lasing 

modes, resulting in less mode beating and amplitude fluctuations of the laser output

power.

Introduction

Seeding a laser cavity with external radiation has long been known to improve 

some characteristics of broadband laser outputs [1]. Several studies have shown that

retroreflection of a small fraction of the laser output back into the laser cavity as optical 

feedback can reduce the amplitude noise of the output on fast time scales (nanoseconds to 

milliseconds). Noise reduction by retroreflection has been experimentally demonstrated

[2-4] and numerically modeled [5-7] for synchronously-pumped mode-locked lasers. 

Subsequent to this initial work, similar noise reduction was also observed [8] for a multimode continuous-wave (cw) Ar-ion laser. In this note the noise reduction effect of Ref. 8 

was reproduced while monitoring the longitudinal mode structure of the laser with and 

without feedback. This allows changes in the laser frequency content associated with the 

introduction of optical feedback (and the onset of noise reduction) to be observed.

The experimental portion of the present work was performed in the mid-1990s 

during preparations [9-13] for the use of stimulated Raman scattering as a nonintrusive 

supersonic wind tunnel diagnostic [14] at Langley Research Center (LaRC). But the

work was not documented at that time because of various time constraints. More recently

other researchers published comprehensive work [15, 16] performing essentially the same 

study as described here. It was decided to document the unpublished LaRC cursory study

to demonstrate consistency between the results of two different studies: the present LaRC 

work and that of Ref. 8.

Experiments and Results

The experimental setup for the present work is shown in the schematic of Fig.1.

A retroreflecting mirror was positioned one cavity length (170 cm) from the output 

coupler of a large-frame cw Ar-ion laser that was operated in a single line (514 nm)

multi-longitudinal-mode configuration, i.e., without an intra-cavity etalon. Either a phase 

conjugate mirror (PCM) or conventional mirror (CM) can be used to accomplish the 

retroreflection and the associated noise reduction. The PCM used in this work was a 

pooled BaTiO3 crystal. A variable transmission filter (VTF) was used to vary the fraction 

of output light that is seeded back into the laser cavity in order to optimize the noise 
3

reduction. As shown in the figure, the laser output beam was divided with beam splitters 

(B/S), and portions were monitored with low and high speed photodiodes (PD) and a 

scanning Fabry-Perot (FP) etalon with free spectral range = 2 GHz and finesse = 150.

Both PDs and the etalon are monitored with oscilloscopes. The polarization rotator is 

necessary only when using the PCM; it can be removed when using a CM.

A clear reduction of amplitude noise in the time domain signal from the highspeed photodiode is observed when the back reflected beam (i.e. feedback) is admitted 

into the laser cavity by removing a beam block located just in front of the retro-mirror.

Simultaneous with the noise reduction, the number of longitudinal modes that are 

observed (on the scanning FP etalon) to be lasing is significantly reduced. Both of these

results are obtained whether using the PCM or CM as the retroreflector. Translating the 

retro-mirror away from the one-cavity position, by 10 cm or more, eliminates the noise 

reduction for the 170-cm cavity length laser.

An example of the spectral result is shown in Fig. 2 using the PCM as the

retroreflector. In this case about 50 % of the incident beam was reflected back towards 

the laser cavity as a phase conjugate beam. Fig. 2a shows  1.8 free spectral ranges of 

the FP etalon output displayed on an oscilloscope trace, as the FP etalon is scanned over 

the laser spectrum. For Fig. 2a, the beam is blocked just in front of the PCM (i.e. no 

optical feedback into the laser cavity). The horizontal white doubled-headed arrow 

indicates the width of one etalon scan or one complete laser spectrum. In both Figs. 2a 

and 2b the scan is repeated almost for two full etalon free spectral ranges. Thick vertical 

green arrows show the locations of the etalon retraces, where the etalon scan stops and 

starts over again. Fig. 2b shows the laser spectrum with the block removed and the VTF

adjusted for about 8 % transmission in one pass.

The fraction of laser output that is seeded back into the Ar-ion laser cavity as 

feedback in this case is estimated to be about 10-4 after accounting for the large 

reflectivity of the laser output mirror. The oscilloscope time scale (equivalent to

frequency domain for the scanning FP etalon) is the same in Figs. 2a and 2b, where the 

scan is over a frequency range equivalent to about two free spectral ranges of the etalon. 

However, the vertical sensitivity is changed by a factor of five for Fig. 2b relative to 2a.

Green vertical arrows indicate the end of each etalon scan and the voltage retrace back to 

the beginning of the next scan. The hash seen in Fig. 2a is the result of a vast number of 

low intensity longitudinal modes that are lasing during the  4 ms that it took to scan the 

etalon over one free spectral range. In contrast Fig. 2b shows only a few modes lasing 

during a similar 4-ms scans. Each sharp vertical spike in Fig. 2b represents one

longitudinal laser mode, except for the last spike on the extreme right-hand side which is 

an artifact of that etalon retrace. Since each observed mode in Fig. 2b could be lasing at a 

different brief time during the 4-ms scan, the few observed modes are not necessarily

lasing simultaneously.

An example of the time domain result is shown in Fig. 3. In this case, the noise 

reduction and spectral narrowing were accomplished by replacing the PCM with a 

conventional plane mirror. Fig. 3a shows a typical short temporal window of the laser 
4

output power fluctuations from ac-coupled high-speed PD (rise time < 0.5 ns) without 

any feedback into the laser cavity (i.e. retroreflected beam blocked beam). The 

sinusoidal-like wave pattern is an  85 MHz oscillation in the laser output power that is

generated from the mode beating of adjacent longitudinal cavity modes. Harmonics of 85

MHz, due to beating of modes that are not nearest neighbors, are weaker and not 

apparent. In Fig. 3b, with the addition of feedback (i.e., beam block removed), the 

fluctuation at 85 MHz is reduced beyond the detection level of the PD with the same 

vertical sensitivity as in Fig. 3a. Lack of a strong 85 MHz oscillation shows that adjacent 

longitudinal modes are not effectively lasing simultaneously. The laser is probably

switching from one mode to another sequentially, as illustrated by observation of more 

than one mode in the time-averaged (4 ms) etalon scan of Fig. 2b.

The experiment described above with an Ar-ion laser was repeated with a 15-mW 

633-nm He-Ne laser (70-cm mirror separation) and with a 0.2-mW 544-nm He-Ne laser 

(25-cm mirror separation). For both He-Ne Lasers, a reduction of the number of lasing 

modes was again observed simultaneously with an amplitude noise reduction when a 

fraction of the output was retroreflected back into the cavity. In each case, the 

retroreflecting mirror was usually located one cavity length from the laser output mirror.

Finally, noise reduction was also measurable with both types of retroreflecting mirrors,

on both the Ar-ion and the smaller He-Ne Lasers, if the retroreflecting mirror was placed 

at a location corresponding to one half of the mirror separation for the laser.

Summary

The results presented here show that the noise reduction in cw ion-lasers from 

retroreflection schemes such as reported in Ref. 8 occurs due to a reduction in the number 

of longitudinal modes that are lasing. Retroreflection of a portion of the laser output back 

into the laser cavity is equivalent to seeding the laser cavity with all operating modes, but 

the few largest modes dominates over the other slightly weaker modes in the mode 

competition during the non-linear amplification process inside the laser cavity – possibly 

to the extent that only a single mode is lasing at any one brief time. This results in less 

mode beating and amplitude noise to the point of complete elimination of mode beating 

noise if only one mode remains lasing. The results of the present work are in agreement 

with previous studies [15, 16] that tested this same hypothesis with similar and more 

extensive measurements. Similar results of noise reduction have also been obtained by 

retroreflection with cw [17] and mode-locked [18] diode lasers.

If a modest loss in laser power can be tolerated (e.g., a factor of 2 or less) similar 

or better noise reductions can be achieved in ion lasers by inserting an etalon inside the 

laser cavity and forcing the laser to operate smoothly on a single longitudinal mode. In 

this way the noise reduction can be achieved without the external optics necessary for the 

phase conjugate reflection process. However if very little laser power can be sacrificed 

for noise reduction, Ref. 8 demonstrates that only a few percent of the beam was 

necessary to achieve significant noise reductions using a phase conjugate mirror.
5

References

1. F. A. Van Goor, “Stabilization of an AM Mode-Locked TEA CO2 Laser,”Opt. 

Comm. 45, 404-406 (1983).

2. M. C. Nuss, U. Keller, G. T. Harvey, M. S. Heutmaker, and P. R. Smith, 

“Amplitude Noise Reduction of 50 dB in Colliding-Pulse Mode-Locked Dye 

Lasers,” Opt. Lett. 15, 1026-1028 (1990).

3. P. Beaud, J. Q. Bi, W. Hodel, and H. P. Weber, “Experimental Observation of the 

Self-Stabilization of a Synchronously Pumped Dye Laser,” Opt. Comm. 80, 31-36

(1990).

4. N. McCarthy, S. Mailhot, and J.-F. Cormier, “Noise Reduction in a 

Synchronously-Pumped Picosecond Dye Laser with a Phase-Conjugate Mirror,” 

Opt. Comm. 88, 403-407 (1992).

5. J. M. Catherall and G. H. C. New, “Role of Spontaneous Emission in the 

Dynamics of Mode Locking by Synchronous Pumping,” IEEE J. Quant. Electron. 

QE-22, 1593-1599 (1986).

6. S. M. J. Kelly, “Mode-Locking Dynamics of a Laser Coupled to an Empty 

External Cavity,” Opt. Lett. 79, 495-501 (1989).

7. G. H. C. New, “Self-Stabilization of Synchronously Mode-Locked Lasers,” Opt. 

Lett. 15, 1306-1308 (1990).

8. N. McCarthy and D. Gay, “Noise Reduction in an Argon Laser with a PhaseConjugating External Cavity,” Opt. Lett. 16, 1004-1006 (1991).

9. G. C. Herring and M. E. Hillard, Jr., “Noise Amplitude Measurements of SingleMode Lasers at Radio Frequencies,” IEEE J. Quant. Electron. 28, 570-572 (1992).

10. G. C. Herring, B. W. South, and R. J. Exton, “Apparatus Translates Crossed Laser 

Beams,” NASA Tech Briefs 18, 41-42 (1994).

11. G. C. Herring and B. W. South, “Pressure Broadening of VIbrational Raman 

Lines in N2 at Temperatures Below 300 K,” J. Quant. Spectrosc. Radiat. Transfer 

52, 835-840 (1994).

12. G. C. Herring and M. E. Hillard Jr., “Effect of Crossed Beams on Stokes Gain and 

Interaction Length in Stimulated Raman Gain Spectroscopy,” Appl. Opt. 34,

6837-6841 (1995).
6

13. G. C. Herring and M. E. Hillard, Jr., “Stark Broadening Using Crossed Beams in 

Stimulated Raman Spectroscopy of N2,” J. Quant. Spectrosc. Radiat. Transfer 57,

739-744 (1997).

14. G. C. Herring, “Noninvasive Measurement of Velocity, Pressure, and 

Temperature in Unseeded Supersonic Air Vortices,” J. Aircraft 46, 1074-1076 

(2009).

15. D. Gay and N. McCarthy, “Effects of Phase-Conjugate Feedback on the Modal

Content and Noise Characteristics of a cw Argon Ion laser: Experimental 

Results,” Opt. Commun. 193, 197-205 (2001).

16. D. Gay and N. McCarthy, “Modal Content and Noise Characteristics of a cw 

Argon Ion Laser with an Optical Feedback: Numerical Simulations of 

Experimental Results,” Opt. Commun. 249, 273-284 (2005).

17. S. Mailhot and N. McCarthy, “Influence of Phase-Conjugate Optical Feedback on 

the Emission of Visible Low-Power Diode Lasers,” Can. J. of Phys. 71, 429-433

(1993).

18. P. Langlois, D. Gay, N. McCarthy, and M. Piche, “Noise Reduction in a ModeLocked Semiconductor Laser by Coherent Photon Seeding,” Opt. Lett. 23, 114-

116 (1998).
7

20-Watt

Ar-ion

Laser

RetroMirror

1 Laser Cavity Length =170 cm

Polarization

Rotator VTF

High Speed PD

Low Speed PD

BaTiO3

PCM

or CM

B/S

FP Scanning

Etalon CM

Removable

Beam Block

Fig. 1. Schematic of the setup used for observing amplitude and frequency characteristics 

of the laser output when providing optical feedback into an Ar-ion laser cavity.
8

Etalon Retraces

Fig. 2. Frequency spectrum of Ar-ion laser (a) without feedback showing so many lasing 

modes that the spectrum appears as hash and (b) with optical feedback from a BaTiO3

PCM showing a greatly reduced number of modes.
9

With Feedback

No Feedback

Fig. 3. Time trace of Ar-ion laser output (a) without feedback showing 85 MHz 

oscillations due to mode beating between adjacent longitudinal modes and (b) with 

feedback from a conventional mirror showing significantly reduced oscillations.
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Amplitude Noise Reduction of Ion Lasers With Optical Feedback 

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14. ABSTRACT

A reduction in amplitude noise on the output of a multi-mode continuous-wave Ar-ion laser was previously demonstrated 

when a fraction of the output power was retroreflected back into the laser cavity. This result was reproduced in the present 

work and a Fabry-Perot etalon was used to monitor the longitudinal mode structure of the laser. A decrease in the number of 

operating longitudinal cavity modes was observed simultaneously with the introduction of the optical feedback and the onset 

of the amplitude noise reduction. The noise reduction is a result of a reduced number of lasing modes, resulting in less mode 

beating and amplitude fluctuations of the laser output power. 

15. SUBJECT TERMS

amplitude, argon-ion laser, laser, laser spectral narrowing, noise reduction, phase conjugate, retro-reflection

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