MODE-LOCKED LASER
FIELD OF THE INVENTION
This invention relates to a mode-locked laser, and to methods of producing mode-locking in a laser.
BACKGROUND OF THE INVENTION
Mode-locked operation is one of the most important modes of operation of laser systems. The spectrum of a laser output generally consists of a series of laser modes separated in frequency by c/2L where c is the velocity of light and L is the length of the laser cavity. These modes originate from fitting whole numbers of half-waves into the laser cavity and the modes can extend in frequency over the entire linewidth of the lasing medium.
Generally, the modes oscillate with random phases with respect to each other and hence give a random intensity output. The technique of mode-locking is concerned with the control of the mode phases such that at one point in time all the mode amplitudes add in phase to give a large intensity pulse. As the modes evolve the mode amplitudes no longer add in phase and the intensity drops rapidly.
At one cavity round trip time later the modes will again add in phase to give another large intensity pulse. The duration of the pulses is limited by the linewidth of the laser medium but is generally in the picosecond range or less. The repetition rate is determined by the round trip time in the cavity and is generally in the nanosecond range.
There are many applications for mode-locked pulse trains in optical communications, ultrafast electronics, chemical dynamics, biology and nonlinear optics.
Conventional mode-locking techniques fall into two categories; active techniques and passive techniques.
The active technique is by far the most widely used method in modern lasers because of its reliability. In this technique an amplitude or phase modulator based on either the electro-optic or acousto-optic effect is placed in the laser cavity. The modulator is then driven by a radio-frequency source at a frequency corresponding to a multiple or sub-multiple of the cavity mode spacing. This couples the laser modes together such that their phases become related and hence gives rise to mode-locking.
Accurate matching of cavity length and modulator drive frequency is essential for this scheme to work. Control of the r.f. source is therefore critical.
In the passive schemes an absorber (usually a dye) is placed in the cavity. The absorber has the property that at very high intensity it becomes transparent. This behaviour is shown by many materials and, when used in this way, they are called saturable absorbers. In this case the mode-locking is accomplished by the inherent fluctuations in the multimode laser giving rise to random intense spikes. The largest of these spikes will be best transmitted by the saturable absorber and smaller spikes will be partially absorbed. After many round trips only one pulse will exist in the cavity and this pulse will bounce back and forth giving rise to a mode-locked pulse train. Passive mode-locking is not very widely used at present since the process is by nature statistical and unpredictable and the absorber dyes tend to degrade.
Recently there has been widespread interest in new schemes of mode-locking which combine the advantages of both active and passive mode-locking but with few of the disadvantages. These schemes come under the general heading of coupled cavity mode locking. In this case an external cavity containing a nonlinear element is coupled to the main laser cavity. The nonlinearity that is used is the intensity dependent refractive index, sometimes called the optical Kerr effect. If the external cavity is interferometrically matched to a multiple of the laser cavity length this nonlinearity can couple the laser modes together to give mode-locked operation. The nonlinearity is a very strong mode-locking mechanism since it becomes stronger as the pulses become shorter.
A prime disadvantage of coupled cavity schemes is that the available output from the system is diminished by the existence of the external cavity.
Researchers at the University of St. Andrews,
Scotland have reported on the unusual mode-locked operation of a Ti:sapphire laser (D.E. Spence, P.N. Kean and W. Sibbett, Optics Letters 16, 42 (1991). Ti:sapphire provides a solid state tunable laser with an extremely large bandwidth. In a commercial Ti:sapphire laser system with an extended laser cavity it was found that if the cavity was mis-aligned such that higher order transverse (spatial) modes oscillated, the laser would self mode-lock. The pulses produced were very short and stable. By inserting prisms into the cavity to compensate for dispersion it was possible to obtain pulses of less than 100 femtoseconds. It is widely believed that the
Ti:sapphire rod is itself acting as a nonlinear element causing the modes to couple together.
As a further development to this research Coherent
Inc., California, have produced a product based on the observations of the St. Andrews group (D.Negus,
L.Spinelli, N. Goldblatt, G. Feugnet, Conference on Lasers and Electro-Optics, Baltimore, May, 1991). In the
Coherent Inc. scheme the cavity is aligned and the nonlinearity in the Ti:sapphire rod gives rise to an intensity dependent refractive index change which gives rise to an intensity dependent lensing. A slit placed near one of the coupling mirrors encourages the laser to operate in a short pulse mode. Coherent Inc. have termed this effect Kerr lens mode-locking.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved mode-locked laser, and this is achieved, in part at least, through providing in the cavity a non-linear lensing element separate from the lasing or gain medium.
According to the present invention there is provided laser apparatus for providing mode-locked pulse trains comprising:
a pump radiation source: and
a cavity comprising coupling means, a lasing medium, an intra-cavity non-linear lensing element distinct from the lasing medium, focusing means for providing sufficient intensity in the lensing element to produce intensity dependent lensing, and an intra-cavity aperture located at a beam waist produced by the lensing element.
In use, it is believed that the light produced by the lasing medium is reflected between the coupling means and is focused by the focussing means in the lensing element at sufficient intensity to produce an intensity dependent refractive index change which gives rise to an intensity dependent lensing, known as the optical Kerr effect. As the aperture is located at a beam waist, it acts to discriminate against low intensity light and encourages the laser to operate at high intensity, and thus the apparatus provides mode-locked pulse trains. Provision of the aperture permits use of a conventional aligned cavity.
By providing a lensing element distinct from the lasing medium within the cavity the present invention permits the different optimum operating intensities on the lensing element and lasing medium to be utilised without compromise. For optimum performance as a lasing medium good matching of the pump and laser beam sizes is desirable, which means that the laser beam may be of relatively low intensity, whereas the desirable intensity dependent lensing effect requires high intensity.
Preferably, the pump radiation source is a diode-pump source arranged for end pumping the lasing medium, which is preferably of Nd:YLF. The provision of the separate lensing element permits end pumping of the lasing medium with a relatively large area spot.
The lensing element may be formed of any suitable transparent material exhibiting non-linear properties, such as glass or crystalline materials, or organic materials.
The coupling means may be in the form of a totally reflecting coupler provided at the end of the lasing medium and a partially reflecting output coupler located adjacent the aperture or slit. Alternatively, the aperture may be located adjacent the totally reflecting coupler.
Preferably, the focusing means is provided by an arrangement of concave mirrors. A folding mirror also may be provided in the cavity.
A dispersion compensation system may also be provided in the cavity remote from the aperture.
According to a further aspect of the present invention there is provided a method of producing a mode-locked output from a laser comprising, within the laser cavity, focusing the optical output from a lasing medium in a separate non-linear material at or above the threshold intensity necessary to produce a significant optical Kerr effect, and passing the optical output subject to said effect through an aperture located at a beam waist in the cavity.
In use, mode-locking may be initiated by a mechanical peturbation, and once initiated, mode-locking is self sustaining.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of the present invention will be described, by way of example, with reference to the accompanying drawing, which is a schematic diagram of a laser in accordance with a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE DRAWING
The drawing illustrates a laser 10 in accordance with a preferred embodiment of the present invention. The laser 10 utilises a longitudinal end pumping scheme and thus has a pump laser 12 focused on the rear face of the lasing medium, in this case in the form of a laser rod 14. The laser 10 is of an astigmatically compensated folded cavity type and is therefore provided with a concave folding mirror 16. A coupler in the form of a totally reflecting mirror 18 is provided at the end face of the laser rod 14, and an output coupler in the form of a plane partially reflecting mirror 20 is provided adjacent an aperture or slit 22. The laser includes focusing means in the form of two concave mirrors 24, 26 located in a folded arm of the laser and focusing the intra-cavity beam on a non-linear lensing element 28.
Utilising this arrangement it is possible to provide a beam above a threshold intensity in the lensing element to produce an intensity dependent refractive index change which gives rise to an intensity dependent lensing effect known as the optical Kerr effect. By locating the slit 22 at a beam waist produced by the lensing element 28, beam components outside the peak intensity are filtered out to create mode locked pulse trains. The length of the slit 22 extends perpendicularly into the plane of refraction formed at the refracting faces of the element 28, these faces preferably being Brewster angled.
The pumping laser 12 is in the form of a diode laser, and the laser rod 14 is of Nd:YLF. The rod 14 is plane/Brewster cut, and the plane end is coated to provide the mirror 18. Longitudinal end pumping schemes for diode pumped solid state lasers of this type are very efficient. The minimum focused spot from a high powered diode laser requires that the laser mode size in the gain medium of the diode pump system is fairly large, typically in excess of 100 um. To ensure high efficiency matching of spot size between the lasing beam and the pump beam is required.
The non-linearity necessary to achieve self mode locking is achieved by use of the lensing element 28 which is in the form of a rod of glass placed at the intra-cavity focus provided by the mirrors 24, 26. The glass is in the form of a Brewster angled rod of SF57.
A specific example of the laser 10 will now be described. The pump laser 12 comprises two orthogonal 3W
GaALAs diode laser arrays (SDL 2482-P1), which are wavelength selected and temperature controlled for operation around 792 nm, the peak of the absorption of
Nd:YLF. Two 6.5mm focal length lenses with high numerical aperture focus the output of the diodes in their highly divergent planes. The polarisation of one diode is rotated through 900 using a halfwave plate, so that the two orthogonally polarised beams are coupled using a polarising beam splitter. The beam is focused in the other plane using a combination of two 40mm cylindrical lenses placed before the beam splitter and a lOmm cylindrical lens placed before the Nd:YLF rod 14. This arrangement produces a focus on the laser rod of approximately 270 um.
The Nd:YLF rod 14 is plane-Brewster cut for operation on the 1047 nm high gain line of Nd:YLF. The plane end 18 is coated to be highly reflecting at 1047 nm and anti-reflecting at 792 nm. The cavity is completed by a 500 mm radius of curvature folding mirror 16, the two 100 mm radius of curvature mirrors 24, 26 providing the intra-cavity focus, and the plane output coupler 20 of transmission 2%. The glass 28 is a 1 cm Brewster angled rod of SF57 placed at an intra-cavity focus of approximately 30 um. The intra-cavity focus is located near the mid point of a folded arm of the laser. The aperture or slit 22 is placed at the beam waist which is formed near the output coupler 20, for example at a distance of 1 cm or less from coupler 20. The width of the slit 22 is about lmm and, of course, the length of the slit does not matter.
The laser 10 as described above displays excellent stability, with amplitude noise of less than 1 and near constant pulse durations. The output power for both CW and mode locked operation of the laser 10 is 80 mW, for an incident pump power of around 5 Watts on the Nd:YLF rod 14.
In mode locked operation the pulse duration was 6 pico secs and the repetition rate was about 125 MHz.