TECHNICAL NOTE

Photon-echo amplification by an external-cavity amplifier

B. S. Ham and M. K. Kim

We demonstrate as much as 2.5 × 105 amplification of a photon-echo signal using a simple external-cavity amplifier. The method may be useful in regeneration of photon echo and memory refresh in a dynamic photon-echo optical memory system. We propose an image storage system that is capable of regenera­tion and amplification of phase-conjugate images in a feedback loop.

Photon echoes in low-temperature solids have gained attention lately as a possible novel technique of optical data storage and processing. The method is based on the ability of atomic transitions to store the Fourier spectrum of laser temporal and spatial phase structure as a population distribution in the inhomo-geneous absorption profile. By utilizing the fre­quency dimension at each focal volume of the input laser, one can increase the storage density by many orders of magnitude over the conventional optical storage technique. Thus it is possible to store a long train of data pulses1 or a series of multiple two-dimensional images2 in a single focal volume. The storage time is not limited by the excited-state life­time when optical pumping into a ground-state hyper-fine structure can be established, which permits many hours of storage time.3 The holographic geom­etry of backward stimulated echo also permits high­speed optical processing of temporal4 and spatial5

data. The principles of high-speed high-density optical

memory and processing by photon echoes have been proved with the above experiments. However, there are numerous technical and materials problems to be solved before a practical photon-echo optical com­puter can be built. These include the relatively short storage time (microseconds to seconds) and low quantum efficiency (less than 1%). A practical dy­namic memory system based on photon echo will require periodic regeneration of stored data, and the

The authors are with the Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48202.

Received 12 October 1993; revised manuscript received 7 March 1994.

0003-6935/94/204472-03$06.00/0. © 1994 Optical Society of America.

d e m a n d on detector sensitivity can be relaxed with a stronger photon-echo signal. In this paper we dem­onstrate a method that optically amplifies and regen­erates a photon-echo signal using an external-cavity laser amplifier. Using a simple arrangement, we obtained greater than 105 amplification with good signal-to-noise ratio.

Figure 1 shows a schematic of the apparatus for external-cavity amplification of photon echo. The photon-echo excitation pulses are provided by a cw standing-wave dye laser pumped by an argon-ion laser. The 0.8-W output of a cw dye laser is modu­lated by an acousto-optic modulator to produce two 1-μs pulses separated by a 3-μs delay with a 20-ns rise time. The 610.1-nm wavelength of the laser is reso­nant with the 1D2-3H4 transition of Pr3+ ions doped (0.1 at. %) in a YAIO3 crystal immersed in a liquid-heUum cryostat. The emitted photon-echo pulse has power amplitude of ~0.1% of input laser power. The photon-echo pulse passes through the external-cavity amplifier, which is pumped by the second harmonic of a pulsed Nd:YAG laser. The external-cavity amplifier consists of a flowing dye cell (13 mm × 25 mm × 51 mm quartz cell) between two 50/50 beam splitters. The cell is oriented near the Brewster angle with respect to the input signal beam, and the pump beam is focused by a cylindrical lens and normally incident upon the front surface of the cell. The dye is 240 mg/L solution of sulforhoda-mine 640 in methanol. In order to avoid amplified spontaneous emission and laser action at high pump power, we misalign the input signal beam by a small angle with respect to the symmetry axis of the cavity. The amplified signal beam is focused onto a photodi-ode through several apertures. When necessary, a boxcar averager is used for signal averaging, and a quarter-meter monochromator with 0.1-nm resolu­tion is used to study spectral characteristics. The

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Fig. 1. Schematic of apparatus for external-cavity amplification of photon echo: CDL, argon-laser-pumped cw dye laser; AOM, acousto-optic modulator; S, Pr3+:YAIO3 sample in a liquid-helium cryostat; DDG, digital delay generator; YAG, second harmonic of a pulsed Nd:YAG laser; EGA, external-cavity amplifier with a dye cell and beam splitters (BS's); PD, photodiode detector; BOX, boxcar averager. The inset shows the pulse sequence of two 1-μs excita­tion pulses, a 1-μs photon-echo pulse, and a 10-ns amplified photon-echo signal.

timing of the entire experiment is controlled by a digital delay generator at a 10-Hz repetition rate.

To test the setup, we used cw and pulsed dye lasers as input and measured amplification for various configurations. As shown in Fig. 2(a), when a cw dye laser is amplified with an external cavity, the output is approximately linear relative to input of as much as 60 mW with an amplification factor of 105 for pump energy of 45 mJ. When a pulsed dye laser is ampli­fied with an external cavity, the output is again approximately linear with respect to input of as much as 0.8 nJ with an amplification factor of 4 × 104 for pump energy of 25 mJ. We also compared efficiency of side pumping and end pumping (the pump beam is nearly counterpropagating relative to the input signal beam) with single-pass amplification of a pulsed dye laser. At 40-mJ pumping energy the amplification factor is 120 for the side pump and 25 for the end pump. In most cases we find that the output satu­rates or even diminishes when the pump energy exceeds ~ 40 mJ.

Figure 2(b) shows the amplification of photon echo. The circles in Fig. 2(b) represent the case in which the beam splitters are removed and the echo signal is amplified by a single-pass side pump without the cavity. The input photon-echo signal is 34 μW in power amplitude and 1 μs long. The 5-ns pump laser pulse is positioned in the middle of the echo pulse, and we measure the instantaneous power of the amplified portion of the echo pulse. As the pump energy is increased to 70 mJ, amplified echo increases to 3.4 mW, an amplification factor 100. The squares in Fig. 2(b) represent the case in which the photon echo is amplified with the cavity in place. The input echo pulse in this case is 67 μW in amplitude and 1 μs long. As the pump energy is increased to 55 mJ, the amplified echo increases to 17 W and saturates, an amplification factor of 2.5 × 105.

Fig. 2. (a) Amplified power output versus input power from a cw laser with an external cavity. The pump energies are 32 mJ (triangles), 39 mJ (squares), and 45 mJ (circles). (b) Amplified power output versus pump energy for photon-echo input: Input echo power is 34 μW with a single-pass side pump, and output is shown in milliwatts (circles). Input echo power is 67 μW with a multipass extemed cavity, and output is shown in watts.

The single-pass gain is ~ 100 at 50-mJ side-pump energy, implying gain coefficient α = 4.6 cm-1 for a 1-cm length of active medium. The end-pump gain is lower because of the shorter interaction length at the dye concentration used. The gain could be im­proved by optimization with respect to the dye concen­tration and the pump focus. The gain is approxi­mately linear over almost the entire range of the signal input used in our experiments. The multi­pass gain from the external cavity is as large as 2.5 × 105. If we assume that the input signal undergoes five passes through the 30-cm length of the cavity during the 5-ns pump pulse and include the transmis­sion loss through beam splitters, the average gain per pass is ~ 30, much smaller than the single-pass gein. The main cause of the reduced gain is the competition between signal amplification and amplified spontane­ous emission. At higher pump energy the cavity starts laser action and the amplified signal diminishes significantly. The laser action may be better con­trolled and amplification may be significantly im­proved by careful placement of an intracavity aper­ture and relative alignment of the signal beam and the cavity axis.

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Fig. 3. Photon-echo optical memory with phase-conjugate regen­eration and amplification: OBJ, input object spatial patterns; BS, beam splitter; M's, mirrors; L's, lenses; PE, medium for generation of backward-stimulated photon echo; EGA, external-cavity ampli­fier; C, camera.

When the pump pulse time is positioned away from the echo pulse, the amplified signal disappears except for the amplified spontaneous emission, which repre­sents ~ 5% noise in the output. However, the 20-nm spectral width of the amplified spontaneous emission is the same as that of the dye absorption, and the 30-GHz width of the amplified echo is the same as that of the input echo signal, much less than the 0.1-nm resolution of our monochromator. There­fore the signal-to-noise ratio after the monochroma­tor is at least 4000.

Although we did not attempt amplification of pho­ton echo produced with a pulsed dye laser because of the lack of a second pulsed YAG laser, it is easy to infer from the data on pulsed dye laser amplification that one would expect at least 105 amplification. Typical conversion efficiency, the ratio of echo energy over input laser energy, is 10-4 to 10-3; thus one should be able to regenerate photon echo by reinsert­ing an amplified echo signal into the echo medium. Such a scheme may be applied to photon-echo dy­namic memory to refresh stored data periodically.

In a practical photon-echo memory device for storing a long stream of bits, one would use an amplitude-modulated cw laser for data and echo generation; amplification of such a long bit stream would require cw pump laser. This could be accomplished in an arrangement using a rare-earth-doped fiber amplifier. Furthermore, it is well known that the phase-conjugation property of backward stimulated photon echo can be used to store many frames of two-dimensional patterns and to retrieve them holographi-cally. In Fig. 3 we propose a photon-echo optical memory that can store two-dimensional images and regenerate phase-conjugate images with amplification. Any distortion introduced in the amplifier can be undone by the phase-conjugation process. With an appropriate thresholding component inserted into the loop, accumulation of noise can also be prevented. As is also well known, processing of stored images is possible when the pump beams of backward stimu­lated photon echo have spatial patterns. The feed­back loop of this scheme may be used in an associative memory system.6

This research is supported in part by the National Science Foundation under grant ECS-9023746.

References 1. Y. S. Bai and R. Kachru, "Coherent time-domain data storage

with a spread spectrum generated by random biphase shifting," Opt. Lett. 18, 1189-1191 (1993); M. Mitsunaga, R. Yano, and N. Uesugi, "Time- and frequency-domain hybrid optical memory: 1.6-kbit data storage," Opt. Lett. 16, 1890-1892 (1991).

2. M. K. Kim and R. Kachru, "Storage and phase conjugation of multiple images using backward stimulated echoes in Pr3+:LaF3," Opt. Lett. 12, 593-595 (1987).

3. M. K. Eim and R. Kachru, "Multiple-bit long term data storage by backward stimulated echo in Eu3+:YAIO3," Opt. Lett. 14, 423-425 (1989).

4. Y. S. Bai, W. R. Babbitt, N. W. Carlson, and T. W. Mossberg, "Real-time optical waveform convolver/cross correlator," Appl. Phys. Lett. 45, 714-716 (1984).

5. X. A. Shen and R. Kachru, "High-speed pattern recognition by using stimulated echoes," Opt. Lett. 17, 520-522 (1992); E. Y. Xu, S. Kroll, D. L. Heustis, R. Kachru, and M. K. Kim, "Nanosecond image processing using stimulated photon echoes," Opt. Lett. 15, 562-564 (1990).

6. M. N. Belov and E. A. Manykin, "Optical associative memory based on an echo correlator," Opt. Lett. 16, 327-329 (1991).

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