EP1987571A1

EP1987571A1 - Monofrequency intra-cavity frequency-tripled continuous laser

Info

Publication number: EP1987571A1
Application number: EP07718011A
Authority: EP
Inventors: Thierry Georges
Original assignee: OXXIUS
Current assignee: OXXIUS
Priority date: 2006-01-20
Filing date: 2007-01-17
Publication date: 2008-11-05
Also published as: US20100220753A1; WO2007083015A1; FR2896629A1; FR2896629B1

Abstract

The invention relates to a diode-pumped intra-cavity frequency-tripled continuous laser device (D), this device comprising: an amplifying medium (A), a birefringent non-linear medium for frequency doubling (X2) and a birefringent non-linear medium for frequency tripling (X3). This laser furthermore includes a polarizing medium (P) so as to form an intra-cavity birefringent filter or Lyot filter, said Lyot filter being adapted so as to allow monofrequency output emission from said laser device.

Description

"Continuous laser, tripled in frequency in intra-cavity and single frequency."

The present invention relates to a continuous laser device tripled in frequency in intra-cavity, pumped by diode; and comprising an amplifying medium, a non-linear birefringent medium for frequency doubling, and a non-linear birefringent medium for frequency tripling.

It is particularly applicable to the design of ultraviolet (UV) or near-UV (300-380nm) lasers used in confocal microscopy, flow cytometry, cell sorting and CD writing. master ("CD mastering"), or the semiconductor inspection

The frequency tripling of a diode pumped continuous laser requires two non-linear conversion stages (ω + ω and 2ω + ω) and can only be effective within one or at least two resonant cavities. The resonant frequency doubling is possible intra-cavity or in an external cavity, slaved to the frequency of the emission of the laser. In both cases, a single frequency fundamental emission is necessary. In the first case (intra-cavity) it is necessary to eliminate the noise. In the second case it is necessary because the strongly resonant cavities (great fineness) are very narrow spectrally.

The second external cavity stage is very complex if one seeks a double resonance with the fundamental wave and the harmonic wave, because two optical paths (fundamental wave and harmonic wave) must be controlled.

The present invention more particularly relates to intra-cavity tripling which is easier to implement because the resonance of the fundamental wave is automatic. On the other hand, the laser cavity is elongated by the insertion of nonlinear crystals and it is much more difficult to make the laser single frequency.

Document G. Mizell, ^Λλ 355-nm CW emission using a contact-bonded crystal assembly pumped with a 1 watt 808 nm diode, Proc.

SPIE Laser Material Crystal Growth and Nonlinear Materials and Devices, Vol. 3610 (1999), relating to an experimentation of a triplet continuous laser in frequency, but of very low power (200 μW max) and not allowing a single frequency operation.

There are many laser publications tripled in frequency in intracavity but pulse operation only, which at least the advantage of greatly increasing the efficiency of tripling.

Finally, the use of a type II doubling is a source of instability, since any rotation of the crystal greatly modifies the state of polarization of the fundamental wave in the cavity and thus the doubling and tripling efficiency. This phenomenon is called birefringence interference. The object of the present invention is to design a continuous wave (CW) laser for frequency-tripled intra-cavity frequency and operating in single frequency. Another object of the invention is the design of such a laser operating in a stable manner, ie by limiting, if necessary, the phenomenon of birefringence interference. At least one of the aforementioned objectives is achieved with a continuous laser device tripled in frequency in intra-cavity, pumped by diode; said device comprising: an amplifying medium, a non-linear birefringent medium for frequency doubling, and a non-linear birefringent medium for frequency tripling; these media are usually crystals.

According to the invention, the laser device further comprises a polarizing medium arranged so as to constitute, with at least one of the birefringent crystals, a birefringent filter or Lyot filter in an intra-cavity, said Lyot filter being adapted to allow an emission single frequency output of said laser device. Preferably, for proper operation of the Lyot filter, the birefringence axes of the non-linear crystals are not parallel to the axes of the polarizing medium. If they are parallel, a birefringent crystal is inserted between the amplifying medium and the polarizing medium, this birefringent crystal having its birefringence axes preferentially oriented at 45 ° to the axes of the polarizing medium.

The output emission wavelength is in the ultraviolet (UV) range. It is the whole of the resonant cavity that can constitute a Lyot filter. The polarizing medium is advantageously arranged between the amplifying medium and the frequency doubling medium. More specifically, these media are crystals such that: for the amplifying medium: Nd: YAG and Nd: YVO ₄ or any other crystal or glass doped by any rare earth or globally any doped glass or crystal having a transition that may oscillate in a laser cavity, - for the frequency doubling medium: KTP, KNbO ₃ , BBO,

BiBO, and LBO or any other nonlinear crystal adapted to frequency doubling for the frequency tripling medium: BBO, BiBO, LBO or any other nonlinear crystal suitable for frequency tripling. With the laser device according to the invention, using a pump diode with 2.4W at 808nm, it has experimentally obtained a single-frequency operation at 355nm with a power exceeding 5mW.

The other advantage of the Lyot filter is that the emitted wavelength is the one with the lowest losses and therefore it is the one whose polarization at the output of the polarizer is parallel to the axis of least loss. The power distribution between the two axes of the doubling and tripling crystals is therefore perfectly controlled and stable.

Advantageously, the axes of the doubling and frequency tripling medium are oriented substantially between 30 and 60 ° with respect to the axes of the polarizing medium. Preferably, the orientation is 45 °. With such a device, the doubling and tripling crystals can be cut and arranged so as to achieve phase matching ("phase matching" in English ") type I and / or II, without the device becoming unstable. preferred mode of implementation of the invention, the polarizing medium comprises one or two Brewster interfaces (interfaces at an angle between two refractive index medium ni and n ₂ such that the tangent of the angle is equal to the ratio of indices).

In particular, apart from the polarizing medium, all other media are preferably parallel-faced crystals.

The device according to the invention constitutes a monolithic linear resonant cavity. Linear cavities are usually the shortest. This small size allows a separation of the axial mode as wide as possible, which is beneficial for the efficiency of a single frequency operation. The design of the device may be such that each medium comprises an inlet face and an exit face parallel to each other and to the other faces of the others. environments; these faces being orthogonal to the output direction of the triplated laser beam.

Advantageously, the amplifying medium, the polarizing medium and the doubling and tripling frequency mediums are optically contacted with each other, which greatly facilitates obtaining a single frequency emission and also reduces the manufacturing costs. It is therefore not necessary to insert focusing elements to adjust the mode size in the non-linear elements as is done in the prior art.

The good order of magnitude of the Free Spectral Interval (ISL) of the Lyot filter is the emission width Δλ _ern of the amplifying medium (ISL = kΔλ _ern with 0.5 <k <1.5). This ensures that there is almost always a single peak of transmission of the filter in the emission width. In the case where there is a peak on either side of the emission band, a change in the temperature of the birefringent elements is sufficient to favor one of the peaks. The length of the nonlinear crystals is generally optimized according to the output power of the UV. If the ISL obtained is not of the order of magnitude of the emission width, it can be adjusted by an additional birefringent crystal. Indeed, it can further provide a second birefringent element disposed after the polarizing medium, the second birefringent medium being adapted to adjust the Free Spectral Interval (ISL) of the Lyot filter if necessary.

We recall that / _sz where β | and δn, are the thicknesses and index difference of the different birefringent crystals forming the filter. The wavelengths at the top of the filter are Λ, = "'% - ^{At these} wavelengths, the polarization of the fundamental wave at the input of the nonlinear crystals is linear and parallel to the axis of low loss of the polarizer It is therefore the Lyot filter that controls the state of polarization in the nonlinear crystals and thus avoids the interference of birefringence.

According to an advantageous characteristic of the invention, the laser device comprises means for controlling the temperatures of non-linear media. Advantageously, the filter is tuned by an agreement of the temperature of the crystals.

The change in the temperature of the birefringent crystals induces a slight displacement of the modes of the cavity and a variation in general more fast of the central wavelength of the peak λ _m . A finer positioning of the wavelength of the mode in the center of the filter can be obtained by modifying the temperature of the amplifying medium for example. Thus, it is possible to tune the laser to a wavelength and to center the transmission mode on the filter.

As an example, if 5 mm of KTP is used for frequency doubling of 1064 nm and 5 mm of LBO (cut for type I phase tuning for the sum of frequency 1064 nm + 532 nm giving 355 nm), the filter Lyot has an ISL = I.87 nm and a dISL / dT = 95 pm / ° C. This latter value is large in comparison with the wavelength variation of the cavity modes (typically a few pm / ° C).

We tested a laser amplifier composed of a Nd: YVO _4, 1 mm thick and 1% doping, a polarizer formed of two prisms silica separated by an air gap and non-linear above crystals. Monofrequency operation was well observed around 1064 nm and a tunability of the order of 100 pm / ° C was measured.

Furthermore, the laser device comprises: a highly reflective mirror (HR) at the fundamental wavelength, this mirror being disposed on the input face of the amplifying medium; and a highly reflective output mirror (HR) at the fundamental wavelength, which mirror is optionally disposed on the output face of the frequency-tripling non-linear birefringent medium.

The laser device may also comprise: a highly reflective mirror (HR) at the frequency-tripled wave, this mirror being arranged between the two nonlinear birefringent doubling and tripling frequency mediums, this makes it possible to preserve the crystals arranged in upstream of the tripler crystal against UV waves and increase the UV power at the output of the laser; and a highly reflective mirror (HR) at the frequency doubled wave, this mirror being disposed between the polarizing medium and the non-linear frequency doubling birefringent medium.

Other advantages and characteristics of the invention will appear on examining the detailed description of a non-limiting embodiment, and the appended drawings, in which: FIG. 1 is a simplified diagram of a first UV laser according to the invention;

FIG. 2 is a simplified diagram of a second UV laser according to the invention.

FIG. 1 shows a laser according to the invention for an emission of 7mW of single-frequency power at 355nm with a 2.4W pump.

This laser device comprises a pump diode D associated with a focusing element F for guiding the beam emitted by the diode to

808nm to an input side of a crystal amplifier A. The crystal doubler

X2 is disposed between the polarizing element P and the tripler crystal X3. The amplifier crystal, the polarizing element, and the doubling and tripling crystals are optically contacted in this order and in a linear fashion. We took care to insert on each side four mirrors. The mirror M1 at the input of the amplifier crystal A; the mirror M2 at the output of the tripler crystal X3; the mirror M3 between the polarizing element and the doubling crystal; the mirror M4 between the two doubling and tripling crystals.

Four Peltier effects are inserted to control the temperature of the diode T _D , the temperature of the amplifying medium T _A and the temperatures of the nonlinear crystals Ti and T ₂ .

The first Peltier element Pl is in contact with the pump diode assembly D and focusing element F. This first Peltier element makes it possible in particular to control the emission wavelength of the diode and to cool the diode.

The second Peltier element P2 is in contact with the amplifying crystal and the polarizing element P. Its function is to cool the amplifier and can be used to finely adjust the wavelength of the cavity mode.

The third Peltier element P3 is in contact with the doubling crystal X2. The fourth Peltier element P4 is in contact with the X3 tripler crystal. The set is fixed on the same support S.

In the design of Figure 1, the output face being flat, the fundamental beam is at its "waist" (focal point) on the mirror. The beam is thus well focused in the crystal tripler, but it may have strongly diverged in the doubling crystal. It is generally preferable to use a length of crystal tripler a little shorter than the length optimal so as not to degrade the conversion of the fundamental to the second harmonic.

The frequency tripled wave generation is in both directions as soon as part of the harmonic wave is reflected by the mirror M2. It is desirable to prevent this wave (usually located in the UV) from propagating in the other crystals of the laser, as many crystals age in the presence of UV. On the other hand, by adjusting the propagation phase in the tripler crystal (by a temperature adjustment), it is possible to increase the output power of the tripled wave by the insertion of the mirror M3.

The power of the second harmonic in the cavity is increased by inserting the mirror M4, reflector to the harmonic wave and ensuring that the mirror M2 is also reflective at the harmonic wavelength. The cavity between the mirrors M2 and M4 becomes resonant when the propagation phase in a round trip is close to 0 modulo 2% radians. This phase can be regulated by the temperature of the doubling crystal, but especially by the choice of the wavelength emitted.

It is possible to have a single temperature control for the two nonlinear crystals according to FIG. 2. In this FIG. 2, a laser is shown in a very schematic manner for which the non-linear doubler 3 and the tripler 5 crystals are not not directly contiguous ^" to the amplifier 1. The Brewster 2 blade serves as a polarizing element The 1064nm amplifying crystal is a Nd: YV0 ₄ doped at 1.1% and 1mm long The input side of this crystal amplifier 1 is HR treated (highly reflective) at 1064nm (> 99.8%) Brewster 2 is a flat, highly melted silica plate of 1mm, the nonlinear group has 4 elements 3 to 6 which are optically bonded. a 5 mm KTP cut for a type II phase-tuning at 35 ° C. The second crystal 5 is a frequency-tripling crystal Several crystals were tested: LBO crystals cut for a 3 mm type I phase-tuning, 4 mm and 5 mm, e t LBO crystals cut for type II phase tuning of 4mm and 8mm. The LBO crystals are sandwiched between two fused silica plates 4 and 6. The output blade 6 is HR treated at 1064 nm (99.65%) and the transmission at 532 nm and 355 nm are respectively 2 to 7% (depending on the mirror ) and 95%. Input blade 4 is HR treated at 355 nm (98%) to prevent UV emission from entering the KTP crystal. The total cavity length is about 20mm. The polarizing medium, which may be the combination of Nd: YVO ₄ and Brewster's slide, in combination with the birefringent crystals rotated at 45 ° allows to obtain a Lyot filter or birefringent filter. The set is temperature controlled by a three Peltier effect of 2W. This allows tuning the peak of the filter wavelength that can be achieved in a temperature range of 1 to 2K. These two crystals tolerate large temperature variations in phase agreement, which makes it possible to maintain the nonlinear frequency conversion. The laser is pumped by a diode 3W 1 * 100 μm 808 nm. The focusing element F is a GRIN lens. The diode is also temperature controlled by a Peltier effect. The Nd: YVO ₄ amplifier crystal is controlled by a Peltier effect.

The use of Type II frequency doubling is generally discouraged because it causes a problem of birefringence interference. The laser device of FIG. 2 overcomes this problem by proposing a solution for operation in single frequency. The axes of the frequency-doubling crystal 3 of type II in FIG. 2 and the axes of the tripler crystal 5 are aligned at 45 ° with respect to the Brewster angle. The polarization of the Nd: YVO _{4 is} aligned with the Brewster polarization so that the whole of the cavity constitutes a birefringent filter or Lyot filter. The wavelength with 100% transmission is linearly polarized in the Brewster blade and also separated on the two polarization axes of the frequency doubling crystal (maximum frequency doubling efficiency).

With a 5mm LBO crystal cut for a phase I chord, the output power reached 7mW.

This has resulted in a frequency-tripled, frequency-invariant, continuous low-frequency (CW) laser that can reasonably replace the current UV gas-ion lasers.

The table below shows a set of possible crystal configurations. The doubling or tripling efficiency can be 100% when the polarization is optimal. Preferred configurations are not necessarily optimized for maximum frequency conversion, but for better stability and simplicity.

Of course, the invention is not limited to the examples that have just been described and many adjustments can be made to these examples without departing from the scope of the invention.

Claims

1. Frequency tripled laser device in intra-cavity, pumped by diode; this device comprising: an amplifying medium, a non-linear birefringent medium for frequency doubling, a non-linear birefringent medium for frequency tripling, characterized in that it further comprises a polarizing medium arranged so as to form a filter birefringent or intra-cavity Lyot filter, said Lyot filter being adapted to allow a single-frequency emission at the output of said laser device.

2. Laser device according to claim 1, characterized in that the polarizing medium comprises one or two Brewster interfaces.

3. Laser device according to claim 1 or 2, characterized in that the axes of the doubling medium and tripling frequency are oriented substantially between 30 and 60 ° relative to the axes of the polarizing medium.

4. Laser device according to claim 3, characterized in that the orientation is 45 °.

5. Laser device according to any one of the preceding claims, characterized in that it further comprises a first birefringent element disposed after the polarizing medium, whose polarization axes are parallel to those of non-linear crystals, the first medium birefringent being adapted to adjust the Free Spectral Interval (ISL) of the Lyot filter.

6. Laser device according to claim 1 or 1 ₁ characterized in that the axes of the doubling medium and tripling frequency are parallel to the axes of the polarizing medium.

7. Laser device according to claim 6, characterized in that the doubling crystal is cut for a type I phase agreement.

8. Laser device according to claim 6 or 7, characterized in that it comprises a birefringent element disposed between the amplifying medium and the polarizing medium.

9. Laser device according to claim 8, characterized in that said second birefringent element is a birefringent crystal whose axes are rotated at 45 ° of the axes of the polarizing medium.

10. Laser device according to any one of the preceding claims, characterized in that apart from the polarizing medium, all other media are crystals with parallel faces.

11. Laser device according to any one of the preceding claims, characterized in that the output emission wavelength is in the range of ultraviolet (UV).

12. Laser device according to any one of the preceding claims, characterized in that it constitutes a monolithic linear resonant cavity.

13. Laser device according to any one of the preceding claims, characterized in that the amplifying medium, the polarizing medium and the doubling medium and tripling frequency are optically contacted with each other.

14. Laser device according to any one of the preceding claims, characterized in that it comprises means for controlling the temperature of the amplifying medium.

15. Laser device according to any one of the preceding claims, characterized in that it comprises means for controlling the temperatures of non-linear media.

16. Laser device according to any one of the preceding claims, characterized in that the width of the Lyot filter is substantially equal to the transmitting width of the transition of the amplifying medium.

17. Laser device according to any one of the preceding claims, characterized in that it comprises a highly reflective mirror (HR) at the fundamental wavelength, this mirror being disposed on the input side of the amplifying medium.

18. Laser device according to any one of the preceding claims, characterized in that it further comprises a highly reflective output mirror (HR) at the fundamental wavelength, this mirror being disposed on the output face of the medium. non linear frequency doubling birefringent.

19. Laser device according to any one of the preceding claims, characterized in that it further comprises a highly reflective mirror (HR) to the triplet wave, this mirror being disposed between the two non-linear birefringent doubling and radiating mediums. tripling of frequency.

20. Laser device according to any one of the preceding claims, characterized in that it further comprises a highly reflective mirror (HR) frequency tripled wave, this mirror being disposed between the non-linear birefringent doubling medium of frequency and non-linear birefringent frequency tripling medium.

21. Laser device according to any one of the preceding claims, characterized in that it further comprises a highly reflective mirror (HR) to the doubled frequency wave, this mirror being disposed between the polarizing medium and the non-linear medium. frequency doubling birefringent.