WO2012072524A1

WO2012072524A1 - A double-pass tapered laser amplifier

Info

Publication number: WO2012072524A1
Application number: PCT/EP2011/071092
Authority: WO
Inventors: Wolf Von Klitzing; Vasiliki Bolpasi
Original assignee: Foundation Of Research And Technology - Hellas/Forth
Priority date: 2010-11-29
Filing date: 2011-11-26
Publication date: 2012-06-07
Also published as: WO2012072524A8; GR1007458B

Abstract

The invention refers to laser amplifierand to a method to amplify light. The amplifierincludesa gain region (1), the gain region (1) having a wideend (4), a narrow end (5)and a tapered region (2) between the wide end (4) and the narrow end (5). The amplifier further includes optical means (10,11,12) at or close to the narrow end (5)of the gain region (1) to inject light that has been amplified in the gain region (1) back to the said gain region (1) of the laser amplifier, so that the light that is injected from the narrow end (5) back to the said gain region (1) is amplified in the gain region (1) twice. The method to amplify light uses a tapered laser amplifier, which includes a gain region (1), the gain region (1) having a wide end (4), a narrow end (5) and a tapered region (2) between the wide end (4) and the narrow end (5). The method includes the following steps in the order listed: a) injecting the light in the wide end, b) amplifying the light in the gain region, c) reflecting the light,d)re- injecting the light back in the gain region, e) amplifying the light for a second time in the gain region and f) emitting the light from the wide end of the laser amplifier. The laser amplifier may be a tapered laser diode.

Description

A DOUBLE-PASS TAPERED LASER AMPLIFIER

The invention refers to laser amplifier and to a method to amplify light. Due to the risk of the high power density damaging the device it is often difficult to achieve very high power in simple straight wave-guided laser amplifiers. For this reason, i.e. when the power of the emitted laser beam would exceed the one that a single mode laser diode can support, e.g. due to gain saturation or optical power density, some laser amplifiers contain a so- called tapered region, where the amplifying region widens from the end where the light enters towards the end from the where the light exits, in order to reduce the power-density. A laser of this type is described, for example, in Mehuys et al. "2.0 W CW, diffraction-limited tapered amplifier with diode injection" Electronic Letters 28:21 , 1992, pp.1944-1946. Tapered lasers normally contain an input facet (the narrow end of the amplifier) followed by a wave-guide section with a small effective cross section followed by a region of expanding cross section, which is followed by the output facet with a large effective cross section, i.e. the wide end of the amplifier. The expansion region often has a trapezoidal shape. If light propagates from the input side then it expands in the expanding region, thus reducing the optical power density and therefore allowing higher power levels. The light that is to be amplified by these devices enters the amplifier at its narrow end, is amplified and then exits at the wide end of the trapezoidal region. The gain region of a typical tapered amplifier typically consists of a tapered region and often also a straight region. The light to be amplified is injected at the narrow end of the device and it is then amplified in the straight region, if it exists, and then in the tapered region before emitted from the wide end of the amplifier. A coupling lens at the narrow end focuses the light to be amplified such that it matches the mode profile of the tapered amplifier at its narrow end. Typically, a lens collimates the output light on the wide end of the tapered laser amplifier. More lenses can be used for additional beam-shaping purposes. One of the limiting features of these high-power optical amplifiers is their limited gain, which is often limited to a few hundred.

A method to increase the gain is disclosed in US 6,456,429 to guide a laser beam through a rare earth doped glass amplifier using wave-guides and a reflective coating. Document EP 0352974 discloses a device where the light is passed though the amplifier once, rotated in polarization, and then passed through it for a second time with the aim of producing a polarization independent optical amplifier apparatus.

An object of the invention is a method and a light amplification device to obtain a large increase in the gain of amplification in laser amplifiers. A further object is a method and a light amplification device to reduce the input power required to achieve a given output of light. A further object is a method and a light amplification device, which has a reduced dependence of the output power on small fluctuations of the power of the light to be amplified.

A laser amplifier according to the invention includes a gain region, whereby the gain region has a wide end, a narrow end and a tapered region between the wide end and the narrow end. The laser amplifier according to the invention further includes optical means at or close to the narrow end of the gain region to inject light that has been amplified in the gain region from the narrow end back to the said gain region of the laser amplifier, so that the light that is injected from the narrow end back into the said gain region is amplified in the gain region for a second time.

A method to amplify light according to the invention employs a tapered laser amplifier including a gain region, whereby the gain region has a wide end, a narrow end and a tapered region between the wide end and the narrow end. The method according to the invention includes the following steps a) to f) in the order listed: a) injecting the light in the wide end, b) amplifying the light in the gain region, c) reflecting the light, d) re-injecting the light back in the gain region, e) amplifying the light for a second time in the gain region and f) emitting the light from the wide end of the laser amplifier.

In accordance with the invention, the light be amplified is injected into the wide end of the tapered laser diode. After passing through the laser amplifier once, it is sent back into it for a second amplification. The increase in total gain is achieved by double-passing the light beam through the gain region of the amplifier. The reduced sensitivity to fluctuations in the power of the input beam is also achieved by double-passing the light beam through the gain region of the amplifier. A laser amplifier according to the invention simplifies Master Oscillator Power Amplifier (MOPA) laser systems in that the master oscillator needs to provide much less input output power in order to achieve the desired output power. This invention could be used to produce more compact or simpler laser systems possibly at a lower cost. It can also be used to reduce in the intensity fluctuation of the light at the output as compared to the intensity fluctuations of the light at the input. Further to the tapered portion, the gain region of the amplifier may also have a straight region between the tapered region and the narrow end.

Sending back the light for the second amplification may be effected for example with the help of an external mirror, optionally with a collimation lens, or by a reflective coating on the narrow end of the tapered laser amplifier. As the light is amplified twice, the invention requires a reduced amount of seeding light in order to achieve the desired output level.

Some of the light at the narrow end of the amplifier might be sampled and detected with the aim of controlling its intensity, for example in order to protect the tapered laser amplifier from excessive light intensities. The measurement of the intensity of the light that is sampled may be used to control the gain of the laser amplifier or the intensity of the light that is to be amplified. A laser amplifier in accordance with the invention may be a tapered laser diode.

Embodiments of the invention will be described with reference to figures 1 to 9, whereby:

figure 1, figure 2, figure 3, figure 4, figure 5 and figure 6 show different embodiments in accordance with the invention,

figure 7 demonstrates the gain that can be achieved with an amplifier in accordance with the invention,

figure 8 demonstrates the gain that can be achieved with a single pass amplifier, and

figure 9 presents an example of a double-passed tapered laser diode. The tapered laser amplifiers presented in figures 1 to 6 have a first end 4 with a wide cross-section, i.e. the wide end 4, and a second end 5 with a narrow cross-section, i.e. the narrow end 5. Between the wide end 4 and the narrow end 5 there is a gain region 1 with a tapered part 2 and a straight part 3. The straight portion is optional and may or may not exist. The wide end 4, i.e. the first end, has a wide cross-section and the narrow end 5, i.e. the second end has a narrow cross-section. The wide cross-section has a cross- sectional area larger than the cross-sectional area of the narrow cross- section.

In the embodiments according to the invention presented in figures 1 to 6, the light 29 to be amplified is injected, into the wide end 4 of the tapered laser amplifier and it is then amplified as it travels through the gain region 1. After the first amplification the light 9 exits the gain region 1 at its narrow end 5. It is then re-injected back to the said gain region 1 of the laser amplifier, so that the light is amplified for a second time. After the second amplification the amplified light 30 emerges from the wide end 4 of the tapered laser amplifier. The gain region 1 of the amplifier of the embodiments presented in figures 1 to 6 consists of a tapered region 2 and a straight region 3. The straight region 3 is optional and may or may not exist. The small arrows 29 in figures 1 to 6 represent the light that is to be amplified. The large arrows 30 in figures 1 to 6 represent the light that has been amplified by passing the laser amplifier twice. The dotted lines 8 indicate the beam of light that contains both the light 29 that will enter the amplifier in order to be amplified and the light 30 that has been amplified and exits from the wide end of the amplifier. The dotted lines 9 indicate the light that has been amplified once in the gain region before amplified in the gain region for a second time.

In the embodiment shown in figure 1 , the light 29 to be amplified is injected into the tapered amplifier from its wide end 4, where it is amplified for a first time. The light 9 then exits the tapered laser amplifier on its narrow end 5, after which it is collimated by a lens 7 and retro-reflected by an external mirror 10, which is close to the narrow end 5 of the gain region. Then the lens 7 focuses the beam onto the narrow end 5 and thus back into the tapered laser amplifier. The light then emerges after a second amplification from the wide end 4 of the tapered laser amplifier. Then it is typically collimated by beam shaping optics such as lenses 6. Other optical elements such as additional lenses, shaping mirrors, or anamorphic prisms may be used instead of or in conjunction with lenses 6 and 7 to shape the light 8, 9. Also, acousto-optic or electro-optic modulators may be used to control the frequency and intensity of the light 9.

In the embodiment shown in figure 2, the light 29 to be amplified is injected into the tapered amplifier from its wide end 4, where it is amplified for a first time. After this it exits the tapered laser amplifier on its narrow end 5, after which it is retro-reflected by an external concave mirror 11 , which is close to the narrow end 5 of the gain region and which has a shape such that the spatial distribution of the returned light 9 matches at least partially the transverse mode profile of the amplifier at its narrow end 5. The light 9 is then amplified for a second time in the tapered amplifier after which it emerges from the wide end 4. Typically, it is then collimated by beam shaping optics symbolized here by a lens 6.

Figure 3 shows an embodiment, whereby the light 29 to be amplified is injected into the tapered amplifier from its wide end 4, where it is amplified for a first time. After its first amplification the light is retro-reflected by a reflective coating 12, which is located on the narrow end 5 of the laser amplifier. The light then emerges after a second amplification from the wide end 4. It is typically then collimated by beam shaping optics, such as a lens 6.

One of the main risks in injecting the light 29, which is to be amplified, into the wide end of the tapered laser amplifier is that the maximum sustainable intensity of the light at the narrow end of the laser amplifier might be exceeded. This is avoided by sampling some of the light at the narrow end of the tapered laser amplifier and detecting its intensity using a photo detector. The resulting signal is then used to control the intensity of the light 9 that is emitted from or injected into the narrow end 5. This can be done, for example, by controlling the intensity of the light that is injected at the wide end 4 or by controlling the gain of the laser amplifier. In the case of a tapered diode laser this can be done by changing the electrical current passing through the laser diode. If the measured intensity of the light 9 at the narrow end 5 exceeds safe levels, then an electronic circuit reduces or switches off the abovementioned current. Figures 4 to 6 show embodiments with such a beam sampling at the narrow end of the tapered laser amplifier. Figure 4 shows the amplifier of figure 3, where a part of the light 9 is transmitted through the reflective coating 12 and directed onto a photo detector 14 at the narrow end 5. The embodiment of figure 5 can be based of the one of figure 1 or figure 2 with some of the light passing through the partially reflecting mirror 32, which may be flat or concave. The embodiment of figure 6 is also based of the one of figure 1 or figure 2 with the addition of a partially reflecting surface 16 directing some of the singly amplified light to the photo-detectors 14 and 15. Only one of the two detectors is required.

The graphs shown in figure 7 and figure 8 demonstrate the improvement in gain that can be achieved with diode laser amplifiers according to the invention over the known single-pass amplifiers. Figure 7 demonstrates the gain that can be achieved with an amplifier in accordance with the invention for a number of electrical drive different currents I_TA- Figure 8 shows for comparison the gain that can be achieved in the same device with a single pass only. At high output powers, the gain with a single-pass amplifier, in terms of the ratio of the power of the output beam over the power of the input beam is 20 and the gain with the amplifier in accordance with the invention is 1200. The invention reduces the amount of light that needs to be injected into the device in order to achieve the desired output power. Comparing the graphs of figure 7 and figure 8, it is shown that an amplifier in accordance with the invention requires reduced seeding power in order to achieve the full output power of the tapered diode laser. Furthermore, by comparing the slopes of the curves at high output powers in the graphs of figure 7 and figure 8, it is clear that the invention has a much-reduced sensitivity of the output power to small fluctuations of the input power.

A diagram of a full laser amplifier system can be seen in figure 9. The light 29, which is to be amplified, arrives in a single mode fiber 17. After collimation 18 and optionally after polarization matching by an optional wave plate 19, beam steering mirrors 23 direct the light 29 to the side port 20 of a Faraday isolator 20, 21, 22. Beam steering mirrors 24 and lenses 25, 26 couple the light into the wide end of the tapered laser amplifier 27. The polarization can be adapted with an optional wave plate 28. After its first amplification the light exits the tapered laser amplifier on its narrow end. The light 9 is then collimated by lens 7. It then encounters beam splitter 16, which directs a small amount of the light onto a photo-diode 14, the signal of which is used to control the light intensity at this narrow end of the diode such that it does not exceed the limits stated by the manufacturer of the amplifier for the injection of light into the narrow end of the tapered laser diode. The part of the light 9 emitted from the narrow end of the amplifier, that has passed the beam splitter, travels towards the mirror 10. It is reflected and coupled by lens 7 back into the narrow end of the tapered amplifier diode 27. The light is then amplified and emerges from the wide end of the amplifier. Lenses 26 and 25 then collimate the beam. The light then passes through the Faraday Isolator 22, 21, 20 and is coupled via mirror 33 and fiber coupler 34 into the output fiber 35. The insert in figure 9 shows the tapered amplifier module in more detail. It consists of a solid lower base 36, 37, a Peltier element 38, and a top plate 39. In order to stabilize the temperature of the tapered amplifier, a temperature controller adjusts the current through the Peltier element 38 according to the readings of the temperature sensor 40. The tapered amplifier module 27 is mounted on the upper base plate 39 together with lenses 26 and 7 and the connector for the laser current 13. Although the full laser amplifier system is similar to the embodiment shown in figure 7, any embodiment of the invention and in particular those described in figure 1, figure 2, figure 3, figure 4 and figure 5 may be incorporated in the full laser amplifier system.

Claims

A laser amplifier including a gain region (1 ), the gain region (1 ) having a wide end (4), a narrow end (5) and a tapered region (2) between the wide end (4) and the narrow end (5), characterized in that the laser amplifier includes optical means (10, 11, 12) at or close to the narrow end (5) of the gain region (1 ) to inject light that has been amplified in the gain region (1) back to the said gain region (1 ) of the laser amplifier, so that the light that is injected from the narrow end (5) back to the said gain region (1 ) is amplified in the gain region (1 ) twice.

A laser amplifier according to claim 1 , whereby the optical means is a flat mirror (10) or a concave mirror (11 ), located close to the narrow end (5) to reflect light that exits from the from the narrow end (5) back to the said gain region (1 ).

A laser amplifier according to claim 1 , whereby the optical means is a reflective coating (12) on the narrow end of the gain region (1 ).

A laser amplifier according to claim 1 , whereby further optical means (7) to control the shape, intensity, or frequency of the light is provided between the optical means (10, 11 ) and the narrow end of the gain region

(1 )

A laser amplifier according to claim 1 , whereby the laser amplifier further comprises means of sampling the light travelling through the narrow end (5).

A laser amplifier according to claim 5, whereby the means of sampling is a partially reflective coating (12) on the narrow end of the gain region (1 ).

A laser amplifier according to claim 5, whereby the means of sampling is an additional partially reflecting surface (16) between the optical means (10, 11 ) and the narrow end of the gain region (1 ).

8. A laser amplifier according to any of claims 1 to 7, whereby the gain region (1 ) has a straight region (3) between the tapered region (2) and the narrow end (5). 9. A laser amplifier according to any of claims 1 to 8, whereby the laser amplifier is a tapered laser diode.

10. A method to amplify light using a tapered laser amplifier including a gain region (1 ), the gain region (1 ) having a wide end (4), a narrow end (5) and a tapered region (2) between the wide end (4) and the narrow end (5), characterized in that the method includes the following steps a) to f) in the order listed: a) injecting the light in the wide end, b) amplifying the light in the gain region, c) reflecting the light, d) re-injecting the light back in the gain region, e) amplifying the light for a second time in the gain region and f) emitting the light from the wide end of the laser amplifier.

1 1 . A method to amplify light according to claim 10, whereby the light is sampled before being re-injected in the gain region. 12. A method to amplify light according to claim 11 , whereby the method includes the step of measuring the intensity of the light that is sampled and the intensity that is measured is used to control the intensity of the light that is emitted from or injected into the narrow end (5). 13. A method to amplify light using a tapered laser amplifier according to any of claims 10 to 12, whereby the laser amplifier is a tapered laser diode.