EP2664037A1

EP2664037A1 - Compact single frequency laser

Info

Publication number: EP2664037A1
Application number: EP11855776.8A
Authority: EP
Inventors: Yuri A. BARANNIKOV; Alexey AVDORKHIN
Original assignee: IPG Photonics Corp
Current assignee: IPG Photonics Corp
Priority date: 2011-01-13
Filing date: 2011-01-13
Publication date: 2013-11-20
Also published as: WO2012096664A1; WO2012096664A8; EP2664037A4

Abstract

A single frequency laser system is configured with an elongated housing extending along a longitudinal axis and having opposite axially spaced upstream and downstream ends. The housing encloses a laser chip configured to emit a radiation which propagates along a light path and emitted through the downstream faucet thereof. One or more spaced frequency discriminators are mounted in the housing downstream from the chip so as to define an external resonant cavity with the upstream faucet of the of the laser chip. At least two or more separate thermoelectric coolers ("TEC") are mounted in the housing to affect the chip and discriminators so as to the output of the system at the desired frequency.

Description

COMPACT SINGLE FREQUENCY LASER

BACKGROUND OF THE DISCLOSURE

Field of the Disclosure

[001 ] The present disclosure relates to a light emitti g device provided with a laser diode and delivery fiber.

[002] Relevant Known Art

[003] Laser diode sources are well known in the ait and widely use for a variety of applications. Among a mass of requirements applied to such a source, stabilized temperature, mechanical stability and compactness deserve close attention. The former two requirements are instrumental in a stabilized pump output which is of paramount importance in many laser systems because any deviation from the desired parameters causes a wavelength change. This is unacceptable in many configurations including single frequency laser systems which have an external resonant cavity.

[004] Many known configurations of laser diode source address the wavelength locking issue - some successfully, others not. But the improvement comes at a price: laser diode sources tend to loose the desired compactness.

[005] The US Patent 5,699,377 (" '377") is just an example illustrating certain structural aspects that still need to be addressed. The '377 patent discloses a narrow linewidth laser source having a laser chip with the configuration known as a standard Butterfly diode mount or package. The configuration as disclosed in the patent includes, among others, a laser diode and fiber clip mounted on respective separate thermoelectric coolers ("TEC"). The fiber chip and delivery fiber are located on respective opposite input and output sides of the laser diode. The compactness of the disclosed structure may not be optimal.

[006] The laser diode module is a delicate device exposed to both mechanical and optical loads. The former is a result of environment characterized by mechanical stresses. For example, even a slight human intervention in the vicinity of the output fiber creates sufficient mechanical loads capable of destabilizing the desired wavelength.

Optical loads are typically associated with backlight radiation originated downstream from the laser module and even along the downstream stretch of the delivery or output fiber. While propagating along an optical circuitry, the light tends to impinge on a variety of formations, both desired and undesired, and backreflect. Furthermore, it is not unusual that due to various causes parasitic wavelengths are generated while the light propagates along the circuitry. When any light backreflected, tit may and often does end up in the laser diode cavity which may detrimentally affect the stabilization of the desired wavelength.

[007] A need therefore exists for a laser diode source characterized by an a single frequency stabilized wavelength, mechanical integrity and compactness.

[008] Another need exists for a laser diode source radiating a stabilized single frequency output and characterized by high optical and mechanical-resistant characteristics.

Requiem

SUMMARY OF THE DISCLOSURE

[009] These needs are satisfied by a disclosed laser source. In particular, the source includes a housing enclosing a laser diode which is operative to generate a stabilized single frequency output and partially a delivery fiber guiding the generated light outside the housing.

[010] In accordance with one aspect, the delivery fiber is provided with a wavelength- selective element also enclosed in the housing and defining an external resonant cavity between itself and the downstream faucet of the laser diode. The wavelength selective element may be selected from a fiber Bi-ag giating or volume grating. The laser diode and wavelength selective element are supported by respective spaced apart TECs. The use of separate TECs allows the wavelength selective element to be positioned in a close proximity to the diode which creates a reliable stress-resistant structure within the housing. Furthermore, closely positioned diode and selective element render the source to be compact.

[011] A further aspect of the disclosed structure includes its enhanced optical and mechanical resistance to stresses originated outside the housing. This is realized by having an optical isolator located downstream from wavelength- selective element. Depending on the configuration of the wavelength, the isolator may be configured as either a fiber isolator or a volume configuration. The isolator prevents light from reentering the resonant cavity. Otherwise, since the disclosed source may be part of a high power fiber laser system, the powerful amplified light may reenter the cavity and destroy the laser source. The isolator provides not only for the attenuation of undesired backreflected optical f equencies, but also for dumping mechanical stresses occurred outside the housing of the device.

BRIEF DESCRIPTON OF THE DRAWINGS

[012] The above and other features and advantages will become more readily apparent from the following description accompanied by the drawings, in which:

[013] FIG. 1 is a side elevational view of the disclosed laser source.

[014] FIG. 2 A - 2B are respective optical diagrammatic views of the disclosed illustrating a single-grating structure and a two-grating structure.

[015] FIG. 3 is a diagrammatic view of a volume Bragg Grating ("VBG")

SPECIFIC DESCRIPTION

[016] Reference will now be made in detail to the disclosed system. The drawings are in simplified form and are far from precise scale. The word "couple" and similar terms do not necessarily denote direct and immediate connections, but also include connections through intermediate elements or devices.

[017] FIG. 1 illustrates the disclosed laser source 10 configured with a housing 12 which encloses a ceramic thermo-conducting plate 14 that provides for the mechanical rigidity of the source. Two separate TECs 16 and 18, respectively, are in thermo contact with a laser diode chip 20, which may be of a standard butterfly configuration, and a wavelength selective element or frequency discriminator 22. The use of two separate TECs 16 and 18 having respective separate control circuits 17 and 19, respectively, which may also be mounted witliin housing 12 enhances, on one hand, the stability of the desired central wavelength. On the other hand, the two-TEC structure also enliances the flexibility of a tuning process when a different central wavelength is needed. [018] The laser diode chip 20 is preferably configured as a powerful laser with an output of up to kW levels. The laser diode is configured with two opposite upstream and downstream faucets 30 and 32, respectively, covered by respective anti -reflective coatings (not shown) and defining an internal resonant cavity which generate a narrow band diode chip radiation through its downstream faucet 32. The use of the powerful laser diode chip 20 is highly beneficial to the overall fiber laser system for the following reasons. First, the fiber laser system may radiate a high power output without the necessity of using an optical amplifier. Second, the output light of the fiber laser system is highly coherent due to the inverse relationship between the power and linewidth hv

expressed as Δν , where Δν is a linewidth range, Pout -power output, L is the

LLPout

length of the resonator cavity.

[019] The device 10 further includes a ferrule 24 mounted to a foundation 15 within the housing and configured to receive the upstream end region 34 of fiber 26. The fiber 26 is preferably, but not necessarily, a polarization maintaining (PM) fiber which provides for the desired polarization extinction ratio ("PER") of the output light.

[020] The control circuit 17 driving laser diode chip 20 may have a current drive circuit and current sensor receiving the output of the drive circuit. The sensor is coupled to a temperature correction circuit operative to process the measure current and compare it to a reference value. The output from the temperature correction circuit is coupled to TEC 1 regulating chip 20 so as to generate the laser diode or chip output.

[021] Referring to FIGs. 2A and 2B, frequency discriminator 22 may be a fiber Bragg grating ("FBG") mounted within the housing at a distance L from the upstream faucet of laser diode 20 so as to define the upstream end of the external cavity with upstream faucet 30 of laser 20. The external cavity, thus, refers to a portion of an optical cavity which is external to the internal laser cavity. The discriminator 20 is tunable, i.e., the element in which the particular wavelength of reflected output light may be adjusted to even further narrow the laser chip radiation, as known to one of ordinary skills.

Preferably, as shown, the range of tuning is achieved by applying thermally-induced stress generated by TEC 18. Other tuning techniques may include acoustically induced stress or simply mechanical one. As The length L is selected so as to minimize

c

modehopping and may be determined as Af = , where c- is light speed, neff -

ILneff

effective index of refraction. To prevent the modehopping phenomenon, the modes should be spaced at a distance Af of about 4GHz from one another. Based on the above, the length L should not exceed about 2.5 sm.

[022] FIG. 2B illustrates the configuration of laser light source 10 with two or more FBGs 22 providing even more stable generation of the desired frequency. To provide the single mode generation, the second length L2 can be determined as follows: L2s = LI +

Afmax>Afgrating. For example, for distance LI = 5 sm and Afmax = 8 neffAf max

GHz, L2 is 7.5 sm.

[023] FIG. 3 illustrates an alternative configuration of frequency discriminator 22 structured as a volume holographic Bragg grating ("VBG"), which consists of a periodic phase or absorption perturbation throughout the entire volume of the element. The VBG 22 is a diffractive element operative to diffract only one given wavelength.

[024] The device 10 further includes a pigtailed optical isolator 28 configured to minimize backreflected light propagation into the resonator and improve mechanical resistance of the in-house structure to outside stress. If frequency discriminator 22 is configured as an FBG, device 10 is configured with a fiber optical isolator supporting fiber 26 outside the housing. The isolator 28 may be configured as a polarization mamtaining fiber optic isolator, which achieves low insertion loss, high return loss and high isolation.

[025] If frequency discriminator 22 is a VHG, isolator 28 can be configured as Faraday optical isolator. In this case, isolator 28 is installed within housing 12 upstream from the downstream end of fiber 26. The VHG has a configuration comprising a periodic or aperiodic effective index of refraction and, for example, may have the fringe pattern stored in a holographic material. The fringe pattern, in turn, xomprises fringes of alternating inedices of refraction, or a layered stack of material with alternating indices of refraction. [026] The fiber 26 is encapsulated in silicone which helps achieve the desired coupling of the fiber and discriminator 22 with the support, efficient dumping of mechanical stresses and reliable thermo contact for stabilizing the temperature of frequency discriminator 22. The use of silicone reduces the degassing effect so detrimental to the work of the chip.

[027] It is possible to have the above-disclosed structure without ferrule 24. This configuration would allow FBG 22 to be mounted to the downstream faucet 36 of fiber 26 and have even a better mechanical stability.

[028] Referring to FIG. 4, gain block 40 of system 10 may have a configuration of fiber amplifier including a multimode (MM) fiber 44 fusion spliced between single mode (SM) fibers 42. The MM fiber 44 supports multiple modes at a wavelength different from the lasing wavelength but is configured to guide substantially only a fundamental mode at the lasing wavelength. The cores 46 and 48 of respective MM active and SM passive fibers 44 and 42 are configured so that the fundamental mode spot size of MM fiber 44 substantially matches that of SM fiber 42. Such a configuration amounts to efficient coupling of the fundamental modes of the respective fibers at the splice, which minimizes the possibility of high mode excitation. Depending on the desired output power of laser system 10, the illustrated schematic may additionally have one or more fiber amplifiers.

[029] The foregoing description and examples have been set forth merely to illustrate the disclosure and are not intended to be limiting. Accordingly, disclosure should be construed broadly to include all variation within the scope of the appended claims.

Claims

1. A laser system operative to emit a single frequency output, comprising:

a housing;

a laser chip mounted in the housing and configured to emit a laser radiation through an output faucet of the laser chip along a light path;

a frequency discriminator mounted in the housing downstream from the chip so as to define an external resonant cavity with an upstream faucet of the of the laser chip,

a thermoelectric cooler ("TEC") mounted in the housing and being in thermal communication with the laser chip so as to maintain the laser radiation; and

a frequency regulator mounted in the housing and spaced from the TEC, the frequency regulator being coupled to the frequency discriminator and configured to prevent a shift of the single frequency output.

2. The system of claim 1 further comprising an optical isolator located downstream from the frequency discriminator and operative to prevent backreflected propagation of light originated downstream from the frequency discriminator.

3. The system of claim 2, wherein the frequency discriminator is configured as a fiber Bragg gr ating ("FBG"), the frequency regulator being selected from the group consisting of a TEC, acoustic source and mechanical source.

4. The system of claim 3 further comprising a fiber having an upstream end mounted in the housing and provided with the FBG, and a pigtail fiber isolator coupled to a downstream end of the fiber outside the housing.

5. The system of clahn 4 further comprising a ferrule receiving the upstream end of the fiber in the housing, the FBG being written in the fiber at a distance from the upstream end of the fiber.

6. The system of claim 4, wherein the FBG is written in the upstream end of the fiber.

7. The system of claim 3 further comprising at least one additional FBG written in the fiber.

8. The system of claim 2, wherein the frequency discriminator is configured as a volume Bragg grating ("VBG"), the optical isolator being a non-pigtail optical isolator mounted in the housing downstream from the VBG.

9. The system of claim 2 wherein the isolator has a polarization maintaining

configuration or non-polarization maintaining configuration.

10. The system of claim 1 further comprising control circuits mounted in the housing and driving respective TEC and frequency regulator.

1 1. The system of claim 1 further comprising a gain fiber block located downstream from the housing and receiving the single frequency output, the gain block being configured with an active provided with a multimode core, which is capable of supporting substantially only a fundamental mode, and spaced input and output single mode passive fibers directly spliced to respective opposite ends of the active fiber, the cores of respective active and passive fibers being configured so that a spot size of the fundamental mode substantially matches that one of the passive fiber.