Mode-hop-free single frequency solid state laser

Details Mode-hop-free single frequency solid state laser Mode-hop-free single frequency solid state laser

1999-12-01

2003-09-02

Ip, Paul (Department: 2828)

Coherent light generators

Particular beam control device

Mode discrimination

C372S020000, C372S068000, C372S107000

Reexamination Certificate

active

06614818

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to single-frequency solid state lasers. More particularly, it relates to lasers in which frequency tuning with temperature occurs without mode hops.
BACKGROUND ART
Single-frequency solid state lasers find many applications in coherent communications, laser radars, and as pump lasers for nonlinear optical frequency conversion, among other applications. The frequency of operation of the laser depends on two important features. First, the solid state gain medium has a characteristic frequency response, generating light in a particular continuous frequency range by stimulated emission in response to pumping, often by laser diodes. Second, the laser cavity has a particular total optical path length (product of physical path length and index of refraction) that determines the allowed discrete wavelengths of axial modes—only an integral or half-integral number of wavelengths is allowed. The gain frequency range is a continuous curve, often a Lorentzian distribution, within which many axial modes are allowed.
FIG. 1
illustrates a typical gain envelope supporting many possible axial modes. The spacing of axial modes, known as the “free spectral range” is inversely proportional to cavity length. In general, only the axial modes near the peak gain wavelength can support lasing.
Various techniques have been developed to ensure single-frequency laser operation. The simplest is to use a very small cavity, which provides a large axial mode spacing, and therefore limits the number of modes that will fit under the gain envelope. In general, these monolithic resonators, in which the cavity is defined by the physical boundaries of the crystal itself, exhibit many problems, including poor pump absorption and exclusion of intracavity tuning elements. They also exhibit significant spatial hole burning, a phenomenon caused by overlap of two portions (or directions) of the beam path in the gain medium. The standing-wave patterns resulting from interference of the beams, when only one axial mode oscillates, gives rise to incomplete gain saturation at the nodes. These low-saturation regions exhibit higher gain than the rest of the laser material and preferentially overlap with the standing wave pattern for adjacent axial modes of the resonator, leading to oscillation of two or more modes at once. In response to these problems, birefringent tuning filters and coupled cavity designs are used to provide frequency-dependent loss for mode discrimination over a wide gain bandwidth. These solutions tend to be very lossy and complex. Twisted mode resonators induce circular polarization, eliminating spatial hole burning. Ring cavities eliminate counter-propagating beams altogether using frequency-selective elements to force unidirectional oscillation.
Even when single frequency operation is attainable using one of the above methods, the problem of axial mode hopping is still present. This phenomenon can be understood by considering the graph of FIG.
1
. The shape and horizontal position of the gain curve
10
is a function of temperature. In particular, the peak gain &ugr;
g
has a particular peak gain frequency tuning rate with respect to temperature (d&ugr;
g
/dT), which is usually negative (i.e. increased temperature leads to a decreased frequency). Each axial mode
12
also has a particular tuning rate with respect to temperature, which depends on the optical path length, and therefore on the thermal expansion coefficient and thermo-optic coefficient (refractive index rate of change with temperature) of the cavity elements. In a monolithic laser, in which the entire cavity is defined by the crystal with polished edges, the cavity optical path length is determined solely by properties of the lasing material. These two temperature responses (i.e., peak gain and axial mode) are completely physically independent. In general, the two tuning rates are unequal, with the axial mode tuning rate usually greater for solid state monolithic lasers. As the temperature of the gain medium is increased, the axial mode frequency &ugr;
m
nearest to the peak decreases more quickly than the peak gain frequency &ugr;
g
, and eventually a different axial mode
12
at a higher frequency will be closer to the peak gain. As a result, the laser will “hop” over to that higher frequency mode.
This behavior is illustrated in
FIG. 2
, a graph of axial mode photon energy versus crystal temperature for a monolithic Nd:YAG laser, a common solid state laser. The peak gain temperature shift is given by the overall slope of the curve
20
, while the piece-wise slope is the tuning rate of the axial mode. The discontinuities in the curve are the mode hops, which occur approximately every 4-5 ° C. for a 20 mm long cavity. Obviously, these mode hops are unacceptable for stable laser operation. Since the mode hops illustrated in the curve of
FIG. 2
are reproducible, it is possible to operate a laser at a temperature away from the discontinuities. Often, however, it is either impossible or undesirable to maintain the crystal temperature within such a small temperature range.
The problem of eliminating mode hops has not been solved satisfactorily in solid state lasers. However, it has been addressed with some success in other types of lasers with very different operating conditions. A laser with an actively stabilized etalon for frequency selection is disclosed in U.S. Pat. No. 5,144,632, issued to Thonn. A parameter indicative of the laser's output frequency, either output power or current supplied, is monitored, and the etalon temperature is varied to maintain operation at a desired frequency. This solution has two significant drawbacks that make it unsuitable: it requires a complicated feedback system for frequency stabilization, and it does not really address the problem of mode hops. Rather, it maintains a fixed frequency, keeping operation away from regions in which mode hops occur.
The problem of mode hopping has also been addressed in semiconductor lasers, which have much different operating conditions than solid state lasers. A waveguide DBR laser containing a semiconductor gain element and a waveguide grating functioning as a resonant cavity end reflector is disclosed in U.S. Pat. No. 5,870,417, issued to Verdiell et al. Various methods of suppressing mode hops are addressed in this device. Minimizing the length of the optical cavity increases the free spectral range, making it less likely for mode hops to occur within a relatively narrow temperature range. The device is also designed with particular lengths and thermal conductivities of substrate material so that the optical path length remains constant with an increase in temperature. As the gain element increases in size with increasing temperature, the end reflector moves closer toward the gain element to maintain a constant optical path length. As a result, the mode frequency remains relatively constant and therefore near the peak gain frequency, which has a tuning rate with a lower magnitude. A third solution is to maintain the entire device at a constant temperature, so that the operation frequency is fixed, and the tuning rates become irrelevant. The device can also be maintained at a temperature between mode hops, but not necessarily at a single exact temperature. Note that these solutions prevent the laser from having a useful tuning range. Other methods include changing the refractive index of the gain medium by current pumping in response to temperature changes of the device. These methods require a very uniform and constant temperature throughout the device. Either the temperature is controlled to be within a very narrow range, or the temperature is assumed to remain within a narrow range. Maintaining a constant optical path length ensures that the axial modes do not shift with temperature. Mode hopping is prevented only if the peak gain frequency also remains virtually constant. As discussed below, maintaining a narrow temperature is unsuitable for solid state laser operation.
The problem of mode hopping in s

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