Coherent light generators – Particular beam control device – Q-switch
Reexamination Certificate
1999-02-02
2001-07-17
Davie, James W. (Department: 2874)
Coherent light generators
Particular beam control device
Q-switch
C372S026000, C372S027000
Reexamination Certificate
active
06263004
ABSTRACT:
This invention relates to a Q-switched laser having exactly determinable laser pulses. Thereby, it is possible to provide stabilisation of the repetition rate of a pulsed laser. The invention relates essentially to a Q-switched micro-chip laser.
This kind of laser generates extremely short, high-peak-power laser pulses which could be useful in many applications, for instance in electronic distance measuring devices (EDM). For these applications it is important to generate each short Q-switched laser pulse at an exactly determinable tine. It is also important to be able to generate a pulse train having exactly determinable time intervals between the individual pulses, i.e. the jitter in time between the pulses should be kept at the lowest possible level.
BACKGROUND OF THE INVENTION
Diode pumped solid-state laser is a rapidly growing field. Passively Q-switched micro-chip lasers are particularly interesting as they are able to provide short pulses (<1 ns) with high peak power (kW) for moderate pump powers, from, for instance, a laser diode or the like, using a simple configuration.
A problem with passively Q-switched lasers is the large jitter in repetition rare. The jitter makes this type of laser impossible to use in a number of applications. In these applications the only possibility has been to use actively Q-switched lasers.
Different techniques for reducing the jitter in passively Q-switched lasers have been described and shown, for instance in “A stabilised mnicrochip laser” by M. Arvidsson et al, CLEO Europe 1996. Hamburg, Paper CFH2 and in “Characterization of Passively Q-switched Microchip Lasers for Laser Radar” by W. J. Manderville and K. M. Dinndorf, SPIE, vol. 2748, pp 358.
A passively Q-switched micro-chip laser for producing high-peak-power pulses of light of extremely short duration is also described in U.S. Pat. No. 5,394,413. A saturable absorber prevents the onset of lasing until the average inversion density within the cavity of the Q-switched laser reaches a predetermined value. The configuration of the laser is then such that, at onset of lasing the saturable absorber becomes transparent, i.e. it is said to be bleached, and a Q-switched output pulse having an extremely short length and high peak power is generated. The problem with this kind of Q-switched lasers is that the lasing times are dependent on its dimensions and not controllable in an exact way.
Actively Q-switched lasers, for instance described by Yariv A., “Optical Electronics in Modern Communications” Fifth Edition, pp 227 to 235, or by Wilson J. Et al, “Optoelectronics An Introduction”, Second Edition, pp 226 to 230, demands a very high round trip loss from the active modulator. This loss is in the order of 100%, which requires a high voltage switching device.
However, actively Q-switched lasers, i.e. Q-switched lasers in which the control of the Q-switching is done directly at the Q-switch, for instance by changing the polarisation of the light, have other problems, such as high switching voltages, possibility of multiple pulsing due to piezoelectric effects and large laser cavities due to large size required regarding the active modulator. Actively Q-switched lasers also need fast high voltage switching power supplies to work.
There is a need for a Q-switched laser in a lot of applications having controllable pulsing, small size and not requiring the high voltage switching devices as nornally being required in actively Q-switched lasers.
A Q-switched mode-locked laser is disclosed in U.S. Pat. No, 4,019,156. This “dual modulation” laser is capable of producing transform-limited pulses having a controllable pulse duration and operates as a synchronous driven optical resonator. Intracavity spontaneous emission is gated symmetrically in time by a Pockels cell that is utilised to provide the first Q-switching and then 100% loss modulation in synchronisation with the pulse round trip time in the cavity. The function of the Pockels cell is thus to suppress the spontaneous emission generated by the active medium in order to let the laser be synchronised, to change the pulse length, and thereby synchonize the pulse length, and to provide order mode-locking. It does thus not participate in the very switching function. The function of the Pockels cell is here to provide a stabilisation. The time for each laser pulse could not be chosen at will but is determined by the design of the laser, i.e. the length of the cavity.
SUMMARY OF THE INVENTION
An object of the invention is to provide a Q-switched laser having an exact determinable time for its lasing pulse.
Another object of the invention is to provide a Q-switched laser having an exactly determinable repetition rate with low jitter.
Still another object of the invention is to provide a small sized Q-switched laser having an extremely exact repetition rate and/or extremely exactly determinable times for each emitted laser pulse.
Yet another object of the invention is to provide a Q-switched laser having an exactly controllable time for its lasing pulse but not requiring a high voltage switching device.
The invention relates to a Q-switched laser and device combining the features, and thus the advantages, of both actively and passively Q-switched lasers, preferably microchip-lasers, which is pumped by at least one pump source.
According to the invention a laser is provided with both a saturable absorber and an active modulator. The combined losses of the active modulator and the saturable absorber delays the onset of lasing, thereby creating a large inversion density. By combining the losses from the saturable absorber and the active modulator, the loss from the active modulator can be drastically reduced compared to the loss from an active modulator in an actively Q-switched laser.
At the instant when a laser pulse should be emitted the loss introduced by the active modulator is removed. Then the gain in the laser exceeds the losses and the lasing starts to bleach the saturable absorber giving rise to a laser pulse. Thereby it is possible to define when the laser pulse is to be emitted in a very accurate way.
The spontaneous emission from the laser media can be monitored in order to know the inversion density. This is possible since the spontaneous emission is proportional to the inversion density. The inversion density or the losses in the cavity could then be controlled to ensure that the laser pulse occurs at the right time.
Those two ways of jitter minimizing can be used either alone or both of them combined.
Thus, the invention relates to a Q-switched laser A Q-switched laser, which is pumped by at least one pump source, providing pulses, and comprising a first mirror means at the first end and a second mirror means at the second end, at least one of said mirrors being partically transmittable and the space between said mirrors providing a laser cavity.
According to the invention this Q-switched laser is characterized by
a combination of gain medium means, saturable absorber means and controllable active modulator means between the first and second mirror;
whereby the losses from said controllable active modulator means being less than the loss required for suppressing lasing in the said laser cavity; the losses from said saturable absorber means being less than the loss required for suppressing lasing in the said laser cavity; and the combined losses from said saturable absorber means and said active modulator means being greater than the losses required for suppressing lasing in the said laser cavity during the determined time between pulses;
control means controlling the lasing such that:
in a first stage the combined losses from said saturable absorber means and said active modulator means is present in said laser cavity, setting the threshold inversion density band high enough to suppress lasing; and
in a second stage the loss from said active modulator is instantly removed, lowering the threshold inversion density band to a level lower than the inversion density in the said cavity, resulting in bleaching of said saturable absorber and
Arvidsson Magnus
Hansson Bjorn
Holmgren Martin
Lindstrom Carsten
Davie James W.
Spectra Precision AB
Young & Thompson
LandOfFree
Laser does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Laser, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Laser will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2467360