Chromium laser produces femtosecond pulses

Passive modelocking of Cr4+:YAG with a saturable absorber creates stable ultrafast pulses tunable in the 1.5-µm fiberoptic telecommunications window.

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Chromium laser produces femtosecond pulses

Passive modelocking of Cr4+:YAG with a saturable absorber creates stable ultrafast pulses tunable in the 1.5-µm fiberoptic telecommunications window.

Michael J. Hayduk, Steven T. Johns, and Mark F. Krol

The transition of ultrafast laser sources from laboratory research instruments to turnkey commercial systems has been spurred by recent research progress. In modelocking techniques, a saturable absorber can reliably and efficiently modelock a tetravalent-chromium-doped laser, resulting in tunable femtosecond pulses in the 1.5-µm range (see photo). This spectral region is of significant interest because of its compatibility with the minimum attenuation window of optical fiber for telecommunications.

The use of solid-state lasers for ultrashort pulse generation covering many wavelength regimes has been the center of much attention.1 This work continues to be motivated by new applications of the ultrafast sciences including biomedical diagnostics, surveillance and counterterrorism measures, metrology, and the development of high-speed telecommunication devices. Chromium-doped laser hosts are of significance in the near-infrared spectral region because of their ability to tune through the 1.3- and 1.55-µm transmission windows of optical fiber.2,3 Femtosecond-pulse development in this wavelength region has applications in fiber sensing as well as in the characterization of novel materials and devices for fiber-based interconnects. Numerous eye-safe applications, such as ranging, are also possible at 1.54 µm.

Two chromium-doped hosts can cover the fiberoptic region using tetravalent chromium (Cr4+) as the lasing ion. Chromium-doped forsterite (Cr4+:Mg2SiO4) and Cr4+:YAG have laser parameters similar to those of Ti:sapphire lasers but with the advantage that both have large absorption bands making them compatible with flashlamp or diode pumping. The Cr4+:forsterite medium has exhibited room-temperature continuous-wave (CW) lasing from 1.21 to 1.32 µm and pulsed operation from 1.13 to 1.37 µm; Cr4+:YAG has shown very broad tunability from 1.34 to 1.6 µm. Pulses as short as 53 fs have been generated with Cr4+:YAG using Kerr-lens modelocking (KLM).4 These lasers are an attractive alternative to color-center lasers that must be operated at cryogenic temperatures.

In KLM lasers the intensity-dependent refractive index of the lasing medium is used to modify the transverse beam parameters resulting in intensity-dependent gain modulation. A soft aperture generated by the pump-beam-defined gain profile in the lasing medium or an actual intracavity hard aperture provides the gain modulation. Thus, the process is highly sensitive to cavity alignment and pump-power fluctuations and is inadequate for applications that require long-term stability of the modelocked pulse train.

A more reliable starting and stabilizing modelocking process using a saturable absorber was recently demonstrated.5,6 B. C. Collins of Princeton University and colleagues used a double quantum well embedded in a distributed Bragg reflector to modelock a Cr4+:YAG laser, producing 110-fs pulses at 1541 nm with 70 mW of average output power.

Key to saturable Bragg reflector

Our device features a structure similar to the saturable Bragg reflector (SBR) used by Collings for modelocking, and we have obtained comparable results. However, this femtosecond Cr4+:YAG laser is able to be tuned from 1.488 to 1.535 µm, making it useful for many applications outside the normal spectroscopy realm of optics and chemistry.

The SBR was grown by molecular-beam epitaxy (MBE) on an undoped (100) GaAs substrate using a Varian (Gloucester, MA) Gen-II solid-source MBE system. The total thickness of buffer layer, saturable absorber region, and cap layer were chosen so that a high-index quarter-wave layer is formed, completing the Bragg reflector (see Fig. 1). Width of the Ga0.47In0.53As wells was selected to place the band edge, or heavy hole exciton absorption resonance, at a wavelength of 1500 nm.

Two quantum wells are used for increased nonlinear reflectance and placed near the peak of the electric-field distribution in the first layer of the reflector, resulting in a low quantum-well saturation intensity.6 The peak reflectance is 99.9% centered at 1530 nm, and the SBR high-reflectance regime extends from 1480 to 1575 nm.

The passively modelocked Cr4+:YAG laser resonator is an astigmatically compensated X-cavity similar to that of a standard Ti:sapphire laser (see Fig. 2). The Brewster-angled, cylindrical Cr4+:YAG laser crystal obtained from the POLYUS Research & Development Institute (Moscow, Russia) is wrapped in indium foil and clamped in a brass housing for efficient heat removal. A thermoelectric cooler under the brass housing maintains crystal temperature at 15°C. The laser rod is placed between two highly reflecting (more than 99.9% from 1350 to 1550 nm) focusing mirrors each with a 10-cm radius of curvature (ROC) and a plane-wedged 1% output coupler is used.

The SBR is placed at the focus of another 10-cm ROC high reflector in the high-reflector arm of the cavity. Two fused-silica Brewster-cut prisms in the output arm of the cavity compensate for excess positive group-velocity dispersion introduced by the crystal and mirror coatings. Overall cavity length is approximately 1.58 m, corresponding to a repetition rate of 95 MHz. Laser pumping is at 1064 nm via a CW Nd:YAG laser focused into the gain medium by a 17.5-cm-focal-length, antireflection-coated mode-matching lens.

Dispersion-compensation gains

In CW operation, without the dispersion-compensating prism pair and SBR, 800 mW of TEM00 output power is obtained at 1487 nm for 6.7 W of ab sorbed pump power. With a 0.6-mm-thick quartz birefringent tuning plate, the CW laser is continuously tunable from 1.460 to 1.584 µm. Self-starting modelocking is readily achieved by inserting the SBR into the cavity. Pulse width, optical spectrum, and pulse train are simultaneously monitored to ensure the presence of stable modelocking.

Without the dispersion-compensating prisms and tuning plate in the cavity, pulsewidths on the order of 1 ps are observed, centered at 1.507 µm. The tuning plate cannot be used with the SBR because it appears to limit spectral bandwidth of the Cr4+:YAG laser, resulting in very unstable operation. Insertion of the fused-silica prisms provides the necessary dispersion compensation, and femto sec ond pulses are readily generated. The absorbed pump-power threshold for lasing and self-starting modelocking is 2.7 W.

The position of the SBR relative to the focusing mirror was optimized to produce a pulse train of a stable, single-pulse-per-cavity round-trip. Optimum stability occurs when the average output power is between 40 and 80 mW, which corresponds to an absorbed pump power of roughly 3.0 W. It should be noted that modelocking is stable over long periods of time and even remains so during fluctuations in the Nd:YAG pump power.

Tuning

By inserting a vertical slit into the cavity between the second prism and the output coupler we are able to tune the laser between 1.488 and 1.535 µm while maintaining femtosecond modelocked operation (see Fig. 3). Wide tunability of the laser comes from careful optimization of the cavity (including pump-lens position, folding-mirror spacing, and separation between the focusing mirror and SBR) as it is tuned away from the free-running wavelength of 1.510 µm. Pump power is increased as required to maintain the optimal average output power of 40 to 80 mW. The time-bandwidth product varies from 0.28 to 0.33, indicating nearly transform-limited pulses over the entire tuning range.

It should be noted that modelocking is stable over the entire tuning range except at the very extremes of both the short- and long-wavelength regions. At these wavelengths, modelocking occurs only for brief intervals; tuning range on the short-wavelength side is reduced by widely scattered vibrational and rotational water absorption, having lines present from 1200 to 1700 nm and limiting the available spectral bandwidth to support femtosecond pulses.7 There is a sharp increase in pulsewidth as the Cr4+:YAG laser is tuned past 1.515 µm. Simulations have shown that the linear absorption of the quantum wells and hence the efficiency of the SBR "rolls off" at these longer wavelengths.8 We expect to extend the tunability of the laser to longer wavelengths by simply tailoring the width of the quantum well to place its band edge at a wavelength slightly exceeding 1.5 µm.

Use of the SBR as the modelocking element in the Cr4+:YAG laser allows for a very flexible and reliable system. Its tunability can be tailored throughout the 1.5-µm region, depending upon the specific application. Compactness of this system has been further enhanced by the development of diode-pumped Nd:YVO4 solid-state lasers, which can serve as very effective pump sources, eliminating the bulky, large-frame Nd:YAG pump laser.6,9 Small-footprint, wall-plug voltage requirements and built-in cooling of this new pump source allow for a relatively portable system. Mobility of the Cr4+:YAG laser combined with its tunable femtosecond-pulse generation provide a very convenient ultrafast system that may make its mark in the near future. o

ACKNOWLEDGMENTS

The authors wish to acknowledge R. P. Leavitt and R. Tober of the US Army Research Laboratory (Adelphi, MD) for their growth of the saturable Bragg reflector and C. R. Pollock of Cornell University (Ithaca, NY) for his many fruitful discussions on the operation of chromium-doped lasers.

REFERENCES

1. F. E. Krausz et al., IEEE J. Quantum Electron. QE-28, 2097 (1992).

2. V. Petricevic, S. K. Gayen, and R. R. Alfano, Appl. Phys. Lett. 52, 1040 (1988).

3. N. B. Angeert et al., Sov. J. Quantum Electron. 18, 73 (1988).

4. Y. P. Tong et al., Opt. Lett. 21, 644 (1996).

5. D. Kopf et al., Opt. Lett. 21, 486 (1996).

6. B. C. Collings et al., Opt. Lett. 21, 1171 (Aug. 1, 1996).

7. D. A. Gilmore, P. Vujkovic Cvijin, and G. H. Atkinson, Opt. Commun. 103, 370 (1993).

8. J. P. Theimer, M. Hayduk, M. Krol, and J. W. Haus, "Mode-locked Cr4+ :YAG Laser: Model and Experiment," submitted to Optics Communications.

9. Y. Ishida and K. Naganuma, Opt. Lett. 21, 51 (1996).

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Ultrafast Cr4+:YAG laser produces modelocked tunable pulses, tens of femtoseconds long, from 1.488 to 1.535 µm--the wavelength region of minimal attenuation by optical fiber.

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Figure 1. Saturable Bragg reflector consists of a distributed Bragg reflector with 24.5 periods of 132.1-nm AlAs low-index and 113.3-nm GaAs high-index quarter-wave layers. A 20-nm Al0.48In0.52As buffer layer is grown on to¥of the partial Bragg stack. The saturable absorber region over the buffer layer is a double- quantum-well structure--7-nm Ga0.47In0.53As well /8-nm Al0.48In0.52As barrier /7-nm Ga0.47In0.53As well. The entire structure is capped by a 74.1-nm Al0.48In0.52As layer.

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Figure 2. Self-starting, passively modelocked Cr4+:YAG laser cavity uses the saturable Bragg reflector as the intracavity modelocking element with a fused-silica Brewster prism pair providing dispersion compensation for femtosecond-pulse generation.

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Figure 3. Tailoring of Cr4+:YAG laser quantum-well widths should reduce the roll-offs in full-width-at-half-maximum (FWHM) pulsewidth (squares) and spectral bandwidth (triangles) above approximately 1510 nm and extend the tunability to longer wavelengths.

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