Miniaturized Q-switched diode pumped solid state laser

Abstract: A miniaturized Q-switch (28) is added to the resonant cavity (15) of a compact laser diode pumped solid state laser (10) to produce short high peak power pulses. Q-switching the compact diode pumped solid state lasers takes advantage of the relatively high gain and short cavity length to provide a desirable combination of pulsewidth and pulse energy. Nd:YAG or Nd:YLF are useful solid state laser materials for Q-switching, or other longer lifetime rare earth ions such as erbium or holmium for greater energy storage. The Q-switch (28) is formed of a material such as TeO₂, SF₁₀, or LiNbO₃ with an acousto-optic figure of merit substantially greater than fused silica. Pulsewidths of 10-­ 50 ns are achieved at pulse energies of 10-20 microJoules. Temoo output is easily produced by the compact laser pumped solid state resonator. Applications include materials processing, link blowing in semiconductor memories, marking and scribing, and optical time domain reflectometry.

Description: The invention relates generally to solid state lasers and more particularly to Q-switched lasers for producing high peak power pulses. Q-switching is a technique used to obtain high peak power laser pulses. Q-switching is performed by modulating the losses in a laser cavity. When cavity loss is high, pumping energy is stored in the laser gain medium by building up the population inversion; the high cavity loss prevents laser action from occuring which would deplete the stored energy. The stored energy is then extracted in a high peak power pulse by reducing the cavity losses. The use of an acoustic-optic modulator inside a laser cavity for Q-switching is described in Chang, "Acousto-optic Devices and Application", IEEE Transactions on Sonics and Ultrasonics , Vol. SU-23, No.1, January 1976 on page 17. However, as noted therein nearly all acousto-optic Q-switches are made of fused silica. Another concern mentioned therein is the substantial RF power that must be applied to the Q-switch to prevent lasing. US-A-4653056 published March 24 1987 and US-A-4656635 published April 7 1987 (equivalent to GB-A-2175127) describes a class of solid state lasers which are laser diode pumped in a longitudinal pumping configuration and which can be pumped in the TEMOO mode. These solid state lasers can be made extremely compact. US-A-4665,529 published May 12, 1987 describes a fibre optic coupled longitudinal pumping scheme which provides further miniaturization of these solid state lasers. In these lasers mode matching of the laser diode pumping source to the active mode volume of the laser cavity provides for high gain in a small volume. These solid state lasers are efficient wavelength and mode converters for diode lasers. Diode pumped lasers do not use cooling water and do not rely on broadband excitation sources. Therefore they are not subject to water or flashlamp induced noise and exhibit greatly reduced thermal lensing. The characteristics result in excellent beam pointing stability and pulse-to-pulse stability. The present applicants have recognised that it would be desirable to obtain these advantages in a Q-switched laser, despite the fact that the typical prior art Q-switched solid state laser cannot produce the desired combination of peak power and short pulse width. A long laser rod is pumped by tungsten or arc lamps. A fused silica Q-switch is also placed in the laser cavity and powered from an RF source connected to a LiNbO₃ transducer mounted to the silica substrate. The cavity is relatively long, typically about 1 foot and the Q-switch is large. It should also be noted that US-A-3624545 discloses a diode pumped laser that is Q-switched. To form short pulses, either a short cavity or increased gain in the cavity is required, since pulse width depends on the product of gain and cavity round trip time. For a 1 foot cavity, the round trip time is about two ns. To obtain high gain in the cavity to compensate for the long cavity length, the gain medium must be very highly pumped. Typically several KW of power are supplied to the arc lamp used to pump the laser rod (since pumping efficiency is only about 5%). However the higher gain required to obtain a desired pulse width can result in too high a pulse intensity since the total energy (product of power and pulse width) in the pulse is approximately constant and peak power increases as pulse width decreases. Therefore there is a tradeoff in obtaining the pulsewidth-energy desired. Thus, the usual results are the right peak power but too long a pulse or a sufficiently short pulse at too high peak power. The present applicants consider that it would be desirable to produce a more effective short pulse Q-switched solid state laser which provides a short pulse at low energy by using a short cavity at moderate gain instead of a longer cavity at very high gains. The aforementioned class of solid state lasers, described in the above mentioned patents, allow the resonator to be made small, typically about 1 inch long, while the efficient longitudinal diode-pumped arrangement (about 30% efficient) provides for a reasonable gain. The output can also easily be produced in TEMOO mode which is useful for many applications. Thus the applicants consider that it would be desirable to incorporate a Q-switch into this type of solid state laser so that very short pulses can be produced at desired low energy levels but still with relatively high peak power. It would also be desirable to utilize non-standard materials for the Q-switch in order to produce a miniaturised Q-switch which fits into such a short cavity with low power pumping requirements. According to a first aspect of the present invention there is provided a solid state laser comprising: a solid state laser rod; cavity means defining a compact resonant cavity which contains the laser rod, including output coupling means; laser diode pumping means positioned to pump the laser rod longitudinally and to match the pumping volume to the mode volume in the laser rod; and wherein: an acousto-optic Q-switch is mounted in the resonant cavity, the acousto optic Q-switch being of materials with a high figure of merit and having a short length, thereby to product short high peak power pulses with pulse widths of 50 nanoseconds or less. According to a second aspect of the present invention there is provided a method of producing a laser power pulse, comprising: forming a solid state laser resonator with a resonant cavity containing a solid state laser rod; longitudinally pumping the laser rod with a laser diode pumping source; and matching the pumping volume to the mode volume in the laser rod; wherein: an acoustic diffraction grating is created in an acousto-optic Q-switch in the resonant cavity to prevent lasing while energy is stored in the laser rod, the acousto-optic Q-switch being of materials with a high figure of merit and having a short length; turning off the acoustic diffraction grating to extract the stored energy in the laser rod, a short high peak power pulse, with a pulse width of 50 nanoseconds or less. With the present invention, laser diodes are used to longitudinally end pump a rod of e.g. Nd:YAG or Nd:YLF or some other solid state material so that a small high gain cavity is produced. The cavity can be pumped to produce TEMOO mode. A small acousto-optic Q-switch made of TeO₂, SF₁₀ or LiNbO₃ or other material with a high acousto-optic figure of merit is also placed in the compact cavity so that stored energy in the cavity can be extracted in a short pulse. The Q-switched output has applications for materials processing, link blowing in semiconductor memories, marking and scribing, or optical time domain reflectivity. An embodiment of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Fig 1 is a schematic sectional view of a laser diode pumped Q-switched solid state laser oscillator. Figs 2A to 2C are sectional views of a miniaturised Q-switch in a solid state laser cavity. A diode pumped Q-switched solid state laser oscillator 10 is shown in Fig 1. A solid state laser rod 12 is mounted between a pair of aligned mirrors 14, 16 to form a resonator cavity 15. Mirror 16 is the output coupling mirror which is partly transmitting to the laser radiation but transmissive to the pumping radiation. In an alternate embodiment the mirror 14 may be formed on the end face 18 of the laser rod 12. By separating mirror 14 from the end of laser rod 12 by a few millimetres the laser power is increased about 20% by eliminating spatial hole burning rod. A laser diode pumping source 20 is mounted to provide pumping radiation to laser rod 12. The pumping source 20 is a laser diode or laser diode array. The output of pumping source 20 is collected by a collimating lens 22 which is followed by a focusing lens 24 which longitudinally pumps laser rod 12. Other lenses to correct astigmatism may be placed between lenses 22, 24. In an alternate embodiment, the diode pumping source 20 is not mounted directly to the resonator cavity 15 but may be placed remotely and the pumping radiation transmitted through an optical fiber (as shown in Figure 2A), which is coupled to the resonator cavity 15 to longitudinally end pump the laser rod. These solid state laser configurations are in accordance with the principles and designs of US-A-4,653,056, US-A-656,635 and US-A-4,665,529. An acousto-optic Q-switch 28 is also placed in the cavity 15. Q-switch 28 provides a time dependent loss in cavity 15. An acoustic wave is produced in Q-switch 28 by applying an RF wave to a piezoelectric transducer mounted on a substrate creating a diffraction grating in the material which introduces a loss in the cavity so that rod 12 can be pumped without lasing. When the RF wave is turned off there is no longer an acoustic diffraction grating in the material so that lasing in the cavity occurs with the stored energy being dumped into a short high peak power pulse. In accordance with the principles and teachings of the invention, the solid state laser cavity can be made small and the acousto-optic Q-switch can be made of a material so that it also can be made small so that it fits into the short cavity. In addition, by the longitudinal end pumped pumping scheme, the gain in the short cavity can be relatively high so that very short pulses can be produced at a desired peak power level. The laser rod can be made of Nd:YAG which produces an output of 1.06 microns. The laser rod could alternately be made of Nd:YLF which has a longer fluorescence lifetime than Nd:YAG (480 microsec compared to 230 microsec) so that more energy can be stored. In general, the longer the lifetime of the excited states in the gain material, the greater population inversion can be stored while the Q-switch prevents lasing and the higher energy output will be obtained. Thus, longer lifetime rare earth ions could be used in place of Nd; for example Erbium (Er) or Holmium (Ho) could be used. In order to build a small Q-switch, nonstandard materials must be used. Chang, page 7, Table I, lists a number of acousto-optic materials with their figures of merit. A small Q-switch will allow a shorter laser cavity for shorter pulses and also require lower RF power. The acousto-optic figure of merit is a measure of the extent to which diffraction is produced for a given amount of electrical energy. The higher the figure of merit, the shorter length of material is required. Preferred materials for the Q-switch used in the invention include TeO₂, SF₁₀ and LiNbO₃, but any material with an acousto-optic figure of merit substantially greater than fused silica could be used. An illustrative embodiment of a miniaturized Q-switched solid state laser cavity 30 is shown in Figures 2A to 2C. The Q-switched cavity 30 is contained in a substantially tubular resonator housing 32. The resonator housing 32 has a pair of end portions 34, 36 which extend out from narrow neck portions 38, 40. Output coupling mirror 42 is mounted in end portion 36 of housing 32 while the solid state laser rod 44 and cavity end mirror 46 are mounted in end portion 34. Mirrors 42 and 46 define the optical cavity; be bending the narrow portions 38, 40 of housing 32 optical alignment of the mirrors 42, 46 can be performed. As previously described, mirror 46 can be separate from laser rod 44 or can be formed on the end surface of the rod 44. The resonator housing 32 may be mounted in an outer laser housing 48 which contains fiber connector 50 which couples optical fiber 52 to the housing 48 to longitudinally end pump laser rod 44 from a remote laser diode pump source. The coupling connector 50 maintains fiber 52 in the proper relationship with any associated focusing optics 54 mounted in housing 48, to efficiently pump rod 44. Alternatively, the laser diode pumping source can be mounted directly in housing 48. A miniaturized Q-switch 56 is mounted in the compact laser resonator housing 32. Q-switch 56 is made of a substrate 58 in which is mounted a small (diamond-shaped) piezoelectric transducer 60, typically made of LiNbO₃. A stripline 62 provides electrical connection from a coaxial cable 64 to transducer 60 (through contacts 66). The coaxial cable is connected through impedance matching network 68 to an electrical connector 70 which is connected to a RF power supply 71. Matching network 68 prevents reflections at the load and provides for efficiently driving the piezoelectric transducer 60. The Q-switch substrate 58, as shown in the side and end views of Figures 2B and 2C, is canted or wedge shaped so that acoustic waves propagated through substrate 58 are not reflected back on themselves. The substrate 58 is also surrounded by an L-shaped heat sink 72 which attaches to the housing 32. In operation an RF signal is input through coaxial cable 64 to transducer 60 to generate an acoustic wave which is propagated through substrate 58 to produce a diffraction grating along the optical axis of the solid state laser resonant cavity. When the diffraction grating is present, the losses prevent lasing and allow energy to be stored in the laser rod 44. When the diffraction grating is removed, the laser produces a short high peak power pulse. By Q-switching these small diode pumped solid state oscillators high peak short pulse width output pulses are obtained. For Nd:YAG 10 microJoule energy pulses have been obtained, while Nd:YLF has produced 20 microJoule energy pulses, at repetition rates of 1 Hz to 20 KHz, and with pulse widths of 10-50 ns. These lasers offer exceptional beam pointing stability and pulse energy stability, with beam pointing stability on the order of ± 1% of the diffraction limited spot size over 10 minute time spans, and pulse to pulse stability of ± 3% peak to peak. More stable performance can be achieved by a mode selection technique based on Q-switch control in which one mode is allowed to gain switch before the cavity is opened. By reducing the Q-switch RF drive level, and allowing one CW mode to oscillate, pulse stabilities of ±1.5% are achieved. Addition of an intra-cavity etalon to allow for drift in RF levels yields ± 1% stability (but would require stabilization of the cavity if used over long periods of time). Q-switched diode pumped solid state lasers give pulse widths and power levels useful for a variety of applications. The benefits of high efficiency, extended life (10,000 hours if the diode is operated at -20C), excellent beam pointing stability and pulse energy stability make them particularly attractive. The ability to produce TEMOO mode is also advantageous, particularly for focussing to a small spot without mode structure. One application is materials processing, in particular link blowing in semiconductor memories. It is necessary to focus the beam to a 1 micron spot, for which TEMOO mode is particularly suitable. In order to burn out the aluminum or tungsten silicate links, pulse to pulse stability is required. Another application is marking or scribing on discrete semiconductor components, printed circuit boards, silk screens and a wide variety of other materials. A pulse of about 20 ns could be used to perform optical time domain reflectometry (OTDR) in which a pulse is propagated through a long optical fiber cable to detect defects. Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention.

Claims: A solid state laser comprising: a solid state laser rod (12); cavity means defining a compact resonant cavity (15) which contains the laser rod (12), including output coupling means; laser diode pumping means (20) positioned to pump the laser rod (12) longitudinally and to match the pumping volume to the mode volume in the laser rod (12); and characterised in that: an acousto-optic Q-switch (28) is mounted in the resonant cavity, the acousto-optic Q-switch (28) being of materials with a high figure of merit and having a short length, thereby to produce short high peak power pulses with pulse widths of 50 nanoseconds or less. A laser according to claim 1, wherein the laser rod (12) is of Nd:YAG or Nd:YLF. A laser according to claim 1, wherein the laser rod (12) is of a solid state laser material doped with erbium or holmium. A laser according to any one of the preceding claims, wherein the pumping volume is matched to the mode volume to produce TEMOO output. A laser according to any one the preceding claims, further including an optical fibre (52) connected to the laser diode pumping means (20) and coupling means (50) for connecting the optical fibre (52) to the resonant cavity (15) in a fixed relationship for pumping the laser rod. A laser according to any one of the preceding claims, wherein the acousto-optic Q-switch comprises a substrate (58) of material having an acousto-optic figure of merit substantially greater than fused silica positioned on the optical axis of the resonant cavity (15), a piezoelectric transducer (60) mounted to the substrate (58), and means (62) for connecting the piezoelectric transducer (60) to an RF source to drive the transducer (60) and generate an acousto diffraction grating in the substrate (58). A laser according to claim 6, wherein the substrate is of TeO₂, SF₁₀, or LiNbO₃. A method of producing a laser power pulse, comprising: forming a solid state laser resonator with a resonant cavity (15) containing a solid state laser rod (12); longitudinally pumping the laser rod (12) with a laser diode pumping source (20) ; and matching the pumping volume to the mode volume in the laser rod (12); characterised in that: an acoustic diffraction grating is created in a Q-switch (28) in the resonant cavity (15) to prevent lasing while energy is stored in the laser rod, the Q-switch being of materials with a high figure of merit and having a short length; turning off the acoustic diffraction grating to extract the stored energy in the laser rod, in the form of a short high peak power pulse, with a pulse width of 50 nanoseconds or less. A method according to claim 8 including forming the laser rod from Nd:YAG or Nd:YLF. A method according to claim 8 or claim 9, or including forming the Q-switch (28) from a material having an acousto-optic figure of merit substantially greater than fused silica. A method according to claim 10 including forming the Q-switch from TeO₂, SF₁₀, or LiNbO₃. A method according to claim 11, further comprising forming a plurality of pulses and focusing the pulses on a semiconductor memory to perform link blowing. A method according to claim 11, further comprising forming a plurality of pulses and focusing the pulses on a material to mark or scribe the material. A method according to claim 11, further including propagating a pulse down a long optical fibre to perform optical time domain reflectometry. A method according to any one of claims 8 to 14, including matching the pumping volume to the mode volume to produce TEMOO output.

Inventors: BAER THOMAS MICHAEL, BAER, THOMAS MICHAEL
Applicants: SPECTRA PHYSICS, SPECTRA-PHYSICS INC.
Application year: 1987
Main classification: H01S3
IPC codes: H01S3