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Recent developments relating to the application of the urea crystal to non-linear optics reviewed. The urea crystal has been shown to be a useful material for non-linear optics applications. Urea has been studied within the context of both frequency upconversion1 and, more recently, optical parametric oscillation (0P0).2-4 It is particularly the latter application which will be discussed.
Urea is an organic crystal within the 42m space group class, the same as the ADP ismorphs. It is optically clear from 200 nm to 1.4 μm, which is consistent with parametric oscillation in the visible and near infrared. Its birefringence is approximately twice that of ADP, which leads to an OPO producing light at shorter wavelengths than for most other non-linear crystals. The non-linear coefficient of urea is approximately 2.5 times that of ADP. While a relatively soft crystal, urea can be optically polished to a flatness of less than an optical wavelength using methods similar to those of ADP. The thermal behavior of urea is excellent; the temperature-dependence of the phase-matching angle is much smaller then ADP. Urea is a hygroscopic crystal, a fact which complicates its practical use. Typically, this problem is overcome by immersing the crystal in an index matching liquid such as hexane. The most difficult problem with regards to the use of urea has been and continues to crystal growth. However, high quality urea crystals of length greater than 20 mm in the (110) direction have been grown from solution in the laboratory. Solution growth requires precise temperature control over very long growth times (on the order of one year). Recently, crystal sizes on the order of 1 cm3 have become commercially available. The urea crystal is positive uniaxial, a characteristic which is advantageous for OPO ications. By utilizing type II (o -> o + e) phase-matching and resonating the ordinary wave, the degree of Poynting vector walk-off of the signal from the pump due to double action is significantly reduced. Furthermore, noncritical phase matching is allowed use the effective d coefficient is a maximum at 90°. The urea optical parametric oscillator has generally consisted of a very simple design. Feedback is accomplished by the use of flat dichroic mirrors, which allow for only one of the generated waves to be resonated. The pump wave is collinear with these generated waves. The pump frequency has usually been 355 nm. Frequency tuning is accomplished through the angular rotation of the crystal. The frequency range covered extends from 498 to 1.23 μm, which is obtained with a single crystal and a single set of dichroic mirrors. Never, a small gap between 640 and 790 nm exists in this range; this is due to metrical obstructions of the particular crystals used and is not thought to be intrinsic.) The mirror reflectivity at the resonated wavelength need not be high, as the total number of round trips allowed during the 7 ns pump pulse is small. The linewidth of the oscillator output was measured to be about 1.2 A near 90° phase-matching.
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