Tunable lasers handbook phần 3 pps

84 Charles Freed

r Laser-Cavity Mirrors 7

(4.3~) PFL

Detector 4 (Saturation

Resonance)

FIGURE 8 Graphic illustration of the saturation resonance observed in CO, fluorescence at 4.3

pm. Resonant interaction occurs for v = vo (when k 1’ = 0). The figure shows an internal absorption

cell within the laser cavity. External cells can also be used. (Reprinted with permission from SooHoo

et d. [76]. 0 1985 IEEE.)

In the initial experiments. a short gas cell with a total absorption path of

about 3 cm was placed inside the cavity of each stable CO, laser [72] with a

Brewster angle window separating the cell from the laser g& tube. Pure CO,

gas at various low pressures was introduced inside the sample cell. A sapphire

window at the side of the sample cell allowed the observation of the 4.3-pm

spontaneous emission signal with a liquid-nitrogen-cooled InSb detector. The

detector element was about 1.5 cm from the path of the laser beam in the sample

cell. To reduce the broadband noise caused by background radiation. the detec￾tor placement was chosen to be at the center of curvature of a gold-coated spher￾ical mirror, which was internal to the gas absorption cell. The photograph of the

laser with which the standing-wave saturation resonance was first observed via

the fluorescence signal at 4.3 pm is shown in Fig. 9. More than two orders of

magnitude improvements in signal-to-noise ratios (SNRs) were subsequently

achieved with improved design low-pressure CO, stabilization cells external to

the lasers [73]. One example of such improved design is schematically shown in

Figure 10.

In the improved design, the low-pressure gas cell, the LN,-cooled radiation

collector, and the infrared (IR) detector are all integral partsbf one evacuated

housing assembly. This arrangement minimizes signal absorption by windows

and eliminates all other sources of absorption. Because of the vacuum enclo￾sure. diffusion of other gases into the low-pressure gas reference cell is almost

completely eliminated; therefore, the time period available for continuous use

of the reference gas cell is greatly increased and considerably less time has to

be wasted on repumping and refilling procedures. One LN, fill can last at least

several days.

4 CO, Isotope Lasers and Their Applications 85

FIGURE 9 Two-mirror stable laser with short intracavity cell. This laser was used for the first

demonstration of the standing-wave saturation resonance observed via the 4.3-pm fluorescence signal.

FIGURE 1 0 Schematic illustration of improved external CO, reference gas stabilization cell.

With the improved cells, significantly larger signal collection efficiency

was achieved simultaneously with a great reduction of noise due to background

radiation, which is the primary limit for high-quality InSb photovoltaic detec￾tors. We have evaluated and tested several large-area InSb detectors and deter￾mined that the LN,-cooled background greatly diminished llf noise in addition

to the expected reduction in white noise due to the lower temperature back￾ground radiation.

Figure 11 shows a typical recorder tracing of the observed 4.3-ym intensity

change as the laser frequency is tuned across the 10.59-ym P(20) line profile

86 Charles Freed

fm=260Hz

r = 0.1 aec (single pole)

16.4% DIP Ps1.75W; PO

p = a034 Torr

FIGURE 1 1 Lamb-dip-like appearance of the resonant change in the 4.3-pm fluorescence. The

magnitude of the dip is 16.4% of the 4.391 fluorescence signal. The pressure in the reference cell

was 0.034 Torr and the laser power into the cell was 1.75 W in the I-P(20) transition. A frequency

dither rate of 260 Hz was applied to the piezoelectric mirror tuner.

with a 0.034-Torr pressure of 12C160, absorber gas. The standing-wave satura￾tion resonance appears in the form of a narrow resonant 16.4% “dip” in the 4.3-

pm signal intensity, which emanates from all the collisionally coupled rotational

levels in the entire (OOOl)+(OOO) band. The broad background curve is due to

the laser power variation as the frequency is swept within its oscillation band￾width. Because collision broadening in the CO, absorber is about 7.5 MHzRorr

FWHM [72], in the limit of very low gas cell pressure the linewidth is deter￾mined primarily by power broadening and by the molecular transit time across

the diameter of the incident beam. The potentially great improvements in SNR,

in reduced power and transit-time broadening, and in short-term laser stability

were the motivating factors that led to the choice of stabilizing cells external to

the laser’s optical cavity. The one disadvantage inherent with the use of external

stabilizing cells is that appropriate precautions must be taken to avoid optical

feedback into the lasers to be stabilized.

For frequency reference and long-term stabilization, it is convenient to

obtain the derivative of the 4.3-pm emission signal as a function of frequency.

This 4.3-pm signal derivative may be readily obtained by a small dithering of

the laser frequency as we slowly tune across the resonance in the vicinity of the

absorption-line center frequency. With the use of standard phase-sensitive detec￾tion techniques we can then obtain the 4.3-pm derivative signal to be used as a

frequency discriminator. Figure 12 shows such a 4.3-pm derivative signal as a

function of laser tuning near the center frequency of the 10.59-pm P(20) transi￾tion. The derivative signal in Fig. 12 was obtained by applying a f200-kHz fre￾quency modulation to the laser at a 260-Hz rate. A 1.75-W portion of the laser’s

output was directed into a small external stabilization cell that was filled with

4 CO, Isotope Lasers and Their Applications 87

S/N = -1000

Af = -f 200 kHz

tm = 260 Hz

T = 0.1 we (ringla pole)

Po = 1.m W; P(20); l0.6~

p = 0.034 Torr

FIGURE 12 nance shown in Fig. 11. SNR - 1000, Af - f200 kHz, and t = 0.1 sec (single pole).

Derivative signal at 4.3 pm in the vicinity of the standing-wave saturation reso￾pure CO, to a pressure of 0.034 Torr at room temperature. It is a straightforward

procedure to line-center-stabilize a CO, laser through the use of the 4.3-pm

derivative signal as a frequency discriminant, in conjunction with a phase-sensi￾tive detector. Any deviation from the center frequency of the lasing transition

yields a positive or negative output voltage from the phase-sensitive detector.

This voltage is then utilized as a feedback signal in a servoloop to obtain the

long-term frequency stabilization of the laser output.

Figure 13 shows a block diagram of a two-channel heterodyne calibration

system. In the system, two small, low-pressure, room-temperature C0,-gas ref￾erence cells external to the lasers were used to line-center-stabilize two grating￾controlled stable lasers. The two-channel heterodyne system was used exten￾sively for the measurement and calibration of C0,-isotope laser transitions

[36,37].

Figure 14 shows the spectrum-analyzer display of a typical beat-note of the

system shown in Fig. 13. Note that the SNR is greater than 50 dB at the 24.4 GHz

beat frequency of the two laser transitions with the use of varactor photodiode

detection developed at MIT Lincoln Laboratory [74,75].

Figure 15 illustrates the time-domain frequency stability that we have rou￾tinely achieved with the two-channel heterodyne calibration system by using the

88 Charles Freed

FIGURE 1 3 Block diagram of the two-channel line-center-stabilized C0,-isotope calibration

system. In the figure, wavy and solid lines denote optical and electrical paths, respectively. (Reprinted

with permission from Freed [75]. 0 1982 IEEE.)

-20

-30

52 dB m^ -40 'D

0 9 -50

7-- I -60

-70

-80 4 + 200kHz

FIGURE 14 The 24.4104191-GHz beat note of a 16012CfSO laser I-P(l2) transition and a

l*C1602 laser I-P(6) transition. The power levels into the photodiode were 0.48 mW for the

16012C180 laser and 0.42 mW for the 12C160, laser. The second harmonic of the microwave local

oscillator was generated in the varactor photodiode. The intermediate-frequency noise bandwidth of

the spectrum analyzer was set to 10 kHz.