KR20070104504A - Athermal external cavity laser - Google Patents

Abstract

An athermal external cavity laser is provided to maintain output light power and output wavelength constantly regardless of external temperature without using an additional temperature control component. An athermal external cavity laser includes a semiconductor optical amplifier(100) and an optical fiber(300). The optical fiber comprises a core(320) formed with Bragg grating(340) and a clad(360) surrounding the core. A thermosetting polymer(400) fixes the optical fiber to a ferrule(500) and has a negative thermo-optic coefficient. The clad surrounding a part of the core with the Bragg grating is thinner than the clad in the other part, and the thermosetting polymer surrounds the clad.

Description

Temperature-independent External Cavity Laser

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a passive optical network system of a wavelength division multiplexing (WDM) method, and more particularly, to an external resonance laser as a light source used in a WDM optical subscriber network system. The present invention is derived from the research conducted as part of the IT new growth engine core technology development project of the Ministry of Information and Communication [Task management number: 2005-S-401-02, Title: Development of ultra-fast optical subscriber network technology].

In general, the denser the channel interval, the WDM method has the advantage of significantly increasing the number of channels. As a result, in WDM systems, the tolerances for wavelength errors of light sources that can occur due to external temperature changes are tight, and in most existing WDM systems, temperatures such as thermo-electric coolers (TEC) and thermistors are An expensive butterfly type with integrated control components is used for the Distributed Feedback-Laser Diode (DFB-LD) optical transmission module.

For example, the optical transmission module used in the Dense Wavelength Division Multiplexing (DWDM) method having a channel spacing of 50 GHz or 100 GHz (0.4 nm or 0.8 nm) is used for interference with adjacent channels regardless of external temperature changes. The oscillation wavelength must be stabilized so that crosstalk caused by crosstalk does not exceed the allowable level, and the light output through the pass band of the multiplexing / demultiplexing used in the network must be stabilized. To this end, the company has used a butterfly package with separate temperature control components such as TEC and thermistor to keep the temperature of the optical device constant.

However, since the DFB laser must not only attach temperature control components such as TECs and thermistors, heat sinks, temperature control circuits, etc., but also select an expensive butterfly type package type, the price of the optical transmission module is It is hard to be lowered more than. In addition, in order to highly stabilize the oscillation wavelength and light output, it is necessary to add a monitoring photodetector and a wavelength fixing device. In this case, the price is high, and thus it is difficult to apply to an optical subscriber network whose price competitiveness is the top priority. In order to replace such expensive DFB-LD, an external resonance laser structure is proposed as a light source structure.

Figure 1a is a schematic cross-sectional view of an external resonant laser using a Bragg grating of the conventional TO-CAN package structure.

Referring to FIG. 1A, the external resonant laser includes a core 32 having a semiconductor amplifier 10, a focusing lens 20, and a Bragg grating 34 formed as an amplifying material, and a clad 36 surrounding the core. It includes an optical fiber 30 having a. The Bragg grating 34 of the optical fiber 30 used here is temperature stable, and the rear face 11 of the semiconductor amplifier 10 and the Bragg grating 34 form an external cavity laser (ECL). . In the figure, the length L _cavity of the external resonator forming the external resonator is indicated by both arrows.

The optical fiber 30 in which the Bragg grating 34 is formed is fixed with a thermosetting epoxy 40 to the ferrule 50 to form a thio-can (TO-CAN) package to form the optical fiber structure 60. . The semiconductor amplifier 10 may generally be a high reflection (HR) coating on the rear surface 11, and an anti-reflection (AR) coating on the front surface 12, that is, the emission surface.

On the other hand, the focusing lens 20 is used to improve the optical coupling efficiency of the semiconductor amplifier 10 and the optical fiber 30. End surface 37 is a ((turned at an angle inclined to the vertical plane of the P _optic) reduces the glass reflected (residual reflection) in cross-section. The optical path of the laser light path (P _optic) of the optical fiber 30 has a cavity (70 When external resonance is formed in such a structure, the wavelength reflected by the Bragg grating 34 is oscillated and outputted among the wavelengths satisfying the phase matching condition. .

FIG. 1B is a cross-sectional view of the I-I portion of FIG. 1A and illustrates a structure in which the core 32, the clad 36, the thermosetting epoxy 40, and the ferrule 50 are stacked in a concentric manner.

The wavelength stability of the DFB-LD packaged by TO-CAN packaged external resonance laser is as follows. In the case of DFB lasers whose lattice determines the oscillation wavelength in the semiconductor gain region, the mass production price is cheap but without temperature control, the thermo-optic coefficient (∂n _LD / ∂T) of the semiconductor material is 2.4x10 ^-4 / K degree, the oscillation wavelength is changed by about 0.1 nm per 1 ℃ according to [Equation 1]. Here, n _LD represents the refractive index of the semiconductor material of the DFB laser.

∂λ / ∂T = λ (∂n / ∂T) / n ............ [Equation 1]

In order to improve the wavelength stability of the light source, the wavelength stability may be improved to about 0.01 nm per 1 ° C. in the case of an external resonance laser in which the lattice is not etched into a semiconductor gain material but an optical fiber having a low thermo-optic coefficient.

However, when the external resonance laser oscillates in a single-mode, when looking at the wavelength spectrum according to the temperature, a mode hoping phenomenon occurs in which the oscillation wavelength jumps at every constant external temperature change.

2A to 2C are graphs illustrating a phenomenon in which mode hopping occurs when the external resonance laser oscillates in a single mode.

FIG. 2A is a graph of the external resonance modes 92 determined by the phase matching condition of the external resonance and the reflection spectrum 94 determined by the Bragg grating of the optical fiber, and the external resonance modes within the reflection spectrum 94. It is shown that the m th mode which feels the highest reflectance among the 92 becomes the oscillation mode 90 of the external resonance laser.

FIG. 2B is a graph showing how the external resonance modes 110 and the reflection spectrum 120 change with increasing temperature. When the current is injected for a long time or the temperature of the external resonance laser increases due to an external environment change, FIG. The external resonant modes 92 and the reflection spectrum 94 are changed to the first moving external resonant modes 93 and the first moving reflection spectrum 95, respectively, and thus, the oscillation mode of the external resonant laser ( 90 also changes to the first moving oscillation mode 91.

Here, the degree of movement of the external resonant modes (Δm1) is ∂λ _ECL / ∂T, which is determined by the thermo-optic coefficient of the materials constituting the external resonance laser and the length of the optical path by [Equation 2].

∂λ _ECL / ∂T = λ (∑ (∂n _i / ∂T) L _i ) / ∑∂n _i L _{i ................} [Equation 2]

In addition, since the degree of shift ΔR1 of the reflection spectrum is proportional to the thermo-optic coefficient of the optical waveguide in which the grating is engraved, it moves by ∂λ _WBG / ∂T. Here, the subscript WBG means a waveguide Bragg grating. Therefore, within a certain temperature range, the final output wavelength is shifted from the oscillation mode 90 to the first moving oscillation mode 91 to correspond to the movement degree Δm1 of the external resonance modes.

FIG. 2C illustrates a case in which the temperature of the external resonant laser increases more than in FIG. 2B. The external resonant modes 92 and the reflection spectrum 94 are respectively the second moving external resonant modes 93a and the reflection spectrum ( 95a). However, since the oscillation mode has a larger reflectance felt by the m-1th mode than the reflectance felt by the mth mode, the oscillation mode of the final output wavelength of the external resonance laser does not become the first moving oscillation mode 91a but the second moving oscillation. The mode 96 is entered. This change in oscillation mode in response to external changes (H _m ) is called mode hopping, and this mode hopping phenomenon occurs periodically with external temperature changes.

3 is a graph showing a mode hopping phenomenon and shift of the oscillation wavelength according to temperature, and shows that mode hopping occurs periodically according to temperature change.

Referring to FIG. 3, it can be seen that as the temperature of the external resonant laser increases, the oscillation wavelength gradually moves to a longer wavelength band, and a mode hopping phenomenon occurs every predetermined period 97 determined by [Equation 3].

_{ΔT = δλ ECL / [(dλ} / dT) ECL - (dλ / dT) WBG] ................. [ Equation 3]

The wavelength spacing of the mode hopping is as much as the interval Δλ between the external resonant modes determined by the external resonant optical path. On the other hand, the wavelength change 99 according to the temperature of Bragg grating is ∂λ _WBG / ∂T, and the wavelength change 98 according to the temperature of the actual oscillation wavelength by mode hopping is ∂λ _ECL / ∂T.

In the case of the external resonant laser operating in the single mode, it is known that the output optical power of the external resonant laser suddenly changes by more than 50% in the mode hopping temperature range in which the oscillation wavelength changes rapidly. This change in the output optical power not only drastically degrades the transmission quality of the WDM optical subscriber network, but also reduces the long-term reliability of the device.

In order to solve the mode hopping problem of the external resonator laser operating in the single mode, the reflection spectrum of the Bragg grating should be kept constant regardless of the temperature, and also the external resonant modes should be fixed regardless of the temperature. .

On the other hand, in the case of an external resonant laser operated in a multi-mode, the change of the output optical power due to mode hopping is negligible compared to the case of the single mode. The reason is that the individual oscillation modes undergo a sudden optical power change during mode hopping, but the sum of the output optical powers of the respective modes remains constant. Therefore, in case of the external resonator laser operated in the multi-mode, the oscillation wavelengths to be output should be designed to be independent of the external temperature change. In addition, since the spectrum width is wider than when oscillating in a single mode, when operating in multiple modes, it is affected by dispersion during transmission. Therefore, it is necessary to adjust the interval and the number of oscillating modes to reduce the influence of dispersion. There is.

The technical problem to be achieved by the present invention is to achieve a low cost of the optical transmission module used in the WDM optical subscriber network, the output optical power and the output wavelength can be kept constant regardless of the external temperature without the use of a separate temperature control component. To provide a temperature independent external resonant laser.

In order to achieve the above technical problem, the present invention is a semiconductor amplifier; An optical fiber having a core on which a Bragg grating is formed and a clad surrounding the core; And a thermosetting polymer fixing the optical fiber to a ferrule and having a negative thermo-optic coefficient, wherein the cladding surrounding the core of the Bragg grating is formed to be thinner than the clad of the other portion. A thermosetting polymer provides a temperature independent external resonant laser that surrounds the clad.

In the present invention, the thermosetting polymer may maintain the reflection spectrum of the Bragg grating regardless of temperature, and the output optical power may be maintained regardless of temperature change.

The thickness of the cladding of the Bragg grating portion is 0.2 ~ 0.4 ㎛, the thermosetting polymer of the thermo-optic coefficient is -1 x 10 ^-4 / deg ~ -1.6 x 10 ^-4 / deg and the refractive index may be about 1.43 ~ 1.445 have.

In the present invention, the external resonance laser may include an optical path compensator between the semiconductor amplifier and the optical fiber. The external resonance laser may be a single-mode external resonance laser, and the optical path compensator makes the output wavelength of the external resonance laser temperature independent of the external temperature by making the length of the optical path in which the external resonance is formed. Can be irrelevant.

The thermo-optic coefficient of the optical path compensator is -1 x 10 ^-4 / deg ~ -2.5 x 10 ^-4 / deg, the optical path length of the optical path compensator may be 500 ~ 2000 ㎛. The optical path compensator is coated with anti-reflection (AR) on the front and rear surfaces, and the front and rear surfaces may be inclined at a predetermined angle, for example, 1 to 3 °, with respect to the vertical surface of the optical path.

In the present invention, the cross section of the optical fiber may be antireflective coated or tilted 8 ° in the vertical plane of the optical path, or may be antireflective coated and tilted 8 ° in the vertical plane of the optical path.

The present invention also in order to achieve the above technical problem, a semiconductor amplifier; An optical fiber fixed to a ferrule by a thermosetting polymer; And a thin-film multi-layer (TFML) transmission filter formed between the semiconductor amplifier and the optical fiber.

In the present invention, external resonance is formed between the semiconductor amplifier and the end surface of the optical fiber in the semiconductor amplifier direction, the external resonance laser can make the output optical power independent of temperature changes. The cross section of the optical fiber has a Fresnel reflectivity of 3 to 5%, or a metal having a thickness of 0.1 μm or less is coated on the cross section of the optical fiber so that the Fresnel reflectance is 20 to 50%, or silicon oxide ( SiO ₂ ) or a metal oxide thin film may be coated to have a Fresnel reflectivity of 95% or less.

The TFML transmissive filter may have a structure in which at least two kinds of thin films having different refractive indices and thicknesses are alternately stacked among silicon oxide and metal oxide thin films. The laminated structure is formed on a glass substrate, and the transmission wavelength gradient according to the temperature of the TFML transmissive filter may be less than 0.01 nm / deg, preferably the variation is 0.003 nm / depending on the temperature change. may be less than or equal to deg.

The TFML transmissive filter is AR coated on the front and rear surfaces, and the front and rear surfaces may be inclined at an angle with respect to the vertical surface of the optical path.

In the present invention, the external resonance laser may include an optical path compensator between the semiconductor amplifier and the TFML transmission filter or between the TFML transmission filter and the optical fiber. The external resonance laser may be a single mode external resonance laser, and the optical path compensator may make the output wavelength of the external resonance laser independent of temperature by making the length of the optical path in which the external resonance is formed independent of the external temperature. have.

In the present invention, the external resonant laser includes a focusing lens to improve the optical coupling efficiency of the semiconductor amplifier and the optical fiber, the semiconductor amplifier has an AR coating formed on the front surface of the emission surface of the laser oscillation, A high-reflection (HR) coating is formed on the rear surface, and external resonance may be formed between the Bragg grating and the rear surface of the semiconductor amplifier.

The emission surface of the semiconductor amplifier has a reflectivity of 1 × 10 ⁻³ or less, and includes an optical mode converter having a down-taper structure, and a far-field angle of a beam output from the emission surface. ) May be 25 ° or less in the vertical and horizontal directions.

In order to solve the mode hopping problem, the external resonant laser of the present invention maintains the output optical power by changing the material composition of a waveguide such as a grating-fiber optical fiber or by changing the structure of the waveguide so that the reflection spectrum is independent of temperature. In addition, the external resonance length of the external resonance laser is fixed to fix the external resonance modes irrespective of temperature, thereby keeping the output wavelength constant.

Since the external resonant laser of the present invention can maintain a constant output optical power and output wavelength regardless of the external temperature without using a separate temperature control component, it is possible to maximize the utilization of wavelength resources, which is an advantage of the wavelength division multiplexing method. In addition, the low cost of the light source module can improve the economics of the entire system.

The external resonant laser of the present invention maintains the output optical power by changing the material composition of waveguides such as lattice-engraved optical fibers, by changing the structure of the waveguide or by using a new type of filter to make the reflection spectrum independent of temperature. In addition, by fixing the external resonant length of the external resonant laser to fix the external resonant modes irrespective of the temperature, it is possible to keep the output wavelength constant.

In addition, the external resonant laser of the present invention does not use an expensive temperature control module as in a butterfly package, thereby reducing the cost of the temperature control module as well as the packaging cost, thereby enabling low-cost device fabrication.

Furthermore, the external resonant laser of the present invention can be usefully used as a DWDM system light source that can reduce the channel spacing 10 times or more compared to CWDM because the output wavelength and output optical power is constant regardless of the external temperature.

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the present invention; In the following description, when a component is described as being on top of another component, it may be directly on top of another component, and a third component may be interposed therebetween. In addition, in the drawings, the thickness or size of each component is exaggerated for convenience and clarity of description, and parts irrelevant to the description are omitted. Like numbers refer to like elements in the figures. On the other hand, the terms used are used only for the purpose of illustrating the present invention and are not used to limit the scope of the invention described in the meaning or claims.

In order for the output optical power to be independent of the external temperature change, the reflection spectrum of the Bragg grating must be kept constant. In order to do so, in the description of FIG. 3, the wavelength change 99 according to the Bragg grating temperature, ∂λ _WBG / ∂ T must be lowered. The method of lowering ∂λ _WBG / ∂T can be used to engrave the lattice on materials with low thermo-optic coefficients, or to change the effective refractive index by changing the structure of the optical waveguide to engrave the lattice.

4A and 4B are cross-sectional views of an external resonance laser according to a first embodiment of the present invention, and show an external resonance laser whose output optical power is independent of an external temperature change.

Referring to FIG. 4A, the external resonance laser 1000 according to the present exemplary embodiment includes a core 320 and a core 320 on which a semiconductor amplifier 100, a focusing lens 200, and a Bragg grating 340 are formed as an amplifying material. It includes an optical fiber 300 having a cladding 360 surrounding the.

Here, the thickness t of the clad 360 surrounding the core 320 of the portion where the Bragg grating 340 is formed is thinner than the clad of the other portion. That is, the thickness of the clad etching portion 380 of the clad 360 is formed to be thinner than the rest of the clad 360. In addition, the thermosetting polymer 400 surrounding the cladding 360 has a suitable thermo-optic coefficient, such as a negative thermo-optic coefficient. The thickness t of the clad 360 and the thermo-optic coefficient of the thermosetting polymer 400 will be described in more detail with reference to FIG. 5 below.

The external resonance laser 1000 of the present embodiment has a low-cost TO-CAN package structure, and the length L _cavity of the optical path in which the external resonance is formed is the bra of the optical fiber 300 at the rear surface 110 of the semiconductor amplifier 100. Up to the grating 340. The semiconductor amplifier 100 is preferably an integrated form of a spot size converter (SSC), which serves to improve the coupling efficiency of the active region and the optical fiber in which light is generated when an external current is injected. In addition, the active region of the semiconductor amplifier 100 preferably has a multi-quantum well structure. The light generated when the external current is injected is optically coupled to the optical fiber end surface 370 through the front surface 120, that is, the emission surface.

Meanwhile, the rear surface 110 of the semiconductor amplifier 100 may be HR coated and the front surface 120 may be AR coated. If the exit surface (120), AR coating degree is because an important role in the performance of an external resonator laser, the reflectivity is 10 ^{- 3} or less is a preferred search. In addition, the end surface 370 of the optical fiber 300 may be AR coated or have an inclination of 8 ° on the optical path vertical surface to prevent performance degradation of the device due to residual reflection. Of course, a combination of AR coating and 8 ° inclination is also possible.

The waveguide of the optical mode converter (not shown) of the semiconductor amplifier 100 uses a down-taper structure, so that the size of the optical mode generated in the active region of the semiconductor amplifier 100 is returned to the emission surface 120. As you proceed, gradually increase in size. The dispersion angle or far-field angle of the far-field light distribution output to the exit surface 120 is preferably 25 ° or less in the vertical / horizontal direction.

In general, when the Bragg grating is engraved on the optical fiber, the reflection spectrum of the Bragg grating 340 moves at a long wavelength of about 0.01 nm per 1 ° C. of the external temperature change. To compensate for this, a portion of the cladding 360 of the optical fiber grating is removed and inserted into the ferrule 500, filled with a polymer 400 material having the appropriate thermo-optic coefficient and then fixed. The gap between the cladding 360 of the etched optical fiber and the core 320 of the optical fiber or the thickness t of the etched cladding 360 is the effective refractive index of the optical fiber with respect to the external temperature change with respect to the thermooptic coefficient of the given polymer material. The change is adjusted to be below the appropriate value.

As a result, the external resonant laser of the present embodiment thins the thickness of the clad 360 of the portion where the Bragg grating 340 is formed and fills the periphery with the thermosetting polymer 400 having a negative thermo-optic coefficient. Lower _WBG / ∂T As a result, the reflection spectrum of the Bragg grating can be kept constant, and the output optical power can be made independent of the external temperature change.

FIG. 4B is a cross-sectional view of the II-II part of FIG. 4A, wherein the optical fiber structure 600 has a cladding 360, a thermosetting polymer 400, and a ferrule 500 stacked in a concentric manner around a core 320. Shows. Unlike the related art, as described above, the thickness t of the clad 360 of the Bragg grating portion is thinned and the outer polymer 400 material has an appropriate thermo-optic coefficient.

5A to 5D are graphs showing conditions for Bragg gratings used in the external resonance laser of the first embodiment. A description with reference to FIG. 4 is as follows.

FIG. 5A shows the wavelength change with temperature while appropriately changing the thickness t of the clad 460 formed in the Bragg grating 340 when the refractive index of the polymer 400 is 1.43. 5B shows that the refractive index of the polymer 400 is 1.435, FIG. 5C shows that the polymer 400 has a refractive index of 1.44, and FIG. 5D shows that the polymer 400 has a refractive index of 1.445. Changing t) shows the change of wavelength with temperature. On the other hand, the polymer 400 here uses a material having a negative thermo-optic coefficient, for example, a material having a thermo-optic coefficient of -1x10 ^-4 / deg to -1.6x10 ^-4 / deg.

Through the graphs of FIG. 5, the material of the polymer 400 and the thickness t of the clad 360 that can be used in the external resonator laser of the present invention can be determined. For example, in FIG. 5C, when the refractive index value of the polymer 400 is 1.44, the change in the oscillation wavelength may be reduced to 0.1 nm or less with respect to an external temperature change of 60 ° C. when the thickness t of the clad 360 is 0.4 μm or less. . Preferably, the thickness of the cladding 360 of the Bragg grating portion is appropriately about 0.2 ~ 0.4 ㎛.

The method for removing the clad 360 of the optical fiber with the Bragg grating is preferably mechanical polishing or chemical etching. As an example of chemical etching, an optical fiber is immersed in a hydrofluoric acid (HF) solution or a buffered-oxide etcher (BOE) solution for a suitable time. Since the etching rate of the optical fiber cladding 360 over time is constant, the degree of etching can be easily determined.

The polymer 400 material used may be a material capable of thermosetting or ultraviolet curing. In particular, when using an ultraviolet curable polymer material, since the ferrule 500 material should be transparent to ultraviolet rays, it is preferable to use a glass such as silica or a material having good transmittance in the ultraviolet region.

6 is a cross-sectional view of an external resonant laser according to a second embodiment of the present invention, which shows the structure of an external resonant laser having an output wavelength and an output optical power independent of an external temperature.

Referring to FIG. 6, the external resonance laser 1000 of the present embodiment has a similar basic structure to the external resonance laser of the first embodiment in a structure in which the output optical power is independent of the output power control side, that is, the external temperature. However, in order to maintain the output wavelength irrespective of the external temperature, the external resonant laser 1000 of the present embodiment further includes an optical path compensator 800.

The optical path compensator 800 is formed on the optical path where the external cavity is formed to compensate for the change in the length L _cavity of the optical path according to the external temperature. In other words, by fixing the external resonant length in the single mode external resonator laser to fix the external resonant modes irrespective of temperature, the output wavelength can be kept constant.

Using the equations (2) and (3), the refractive index, thermo-optic coefficient, and optical path length of the optical path compensator 800 may be determined such that mode hopping is suppressed within a required temperature range.

For example, the length of the semiconductor chip 100 is 600um, the thermo-optic coefficient of the semiconductor chip 100 is 2.2x10 ^-4 / deg, the length of the focusing lens 200 is 1000um, the length of the Bragg grating 340 is 4000um, the optical fiber In the case where the thermo-optic coefficient of the core 320 is 0.1x10 ^-4 / deg, and the thermo-optic coefficient of the optical path compensator 800 is -1.5x10 ^-4 / deg, the optical path length of the optical path compensator 800 is Is about 1200um. The thermo-optic coefficient of the material of the optical path compensator 800 is preferably -1.0x10 ^-4 / deg to -2.5x10 ^-4 / deg, and the optical path length of the optical path compensator 800 is about 500um to 2000um. . Here, the optical path length of the optical path compensator 800 may be generally the thickness of the optical path compensator 800, but when the optical path compensator 800 is inclined with respect to the optical path, the path of the path through which the light actually passes. Means length.

Meanwhile, the front surface 820 and the rear surface 810 of the optical path compensator 800 are AR coated, and the optical path compensator 800 is inclined at a constant angle θ with respect to the vertical plane of the optical path. Can reduce residual reflection. The tilt angle θ is preferably about 1 to 3 degrees.

The external resonant laser of the present embodiment is particularly useful for single mode external resonant lasers. That is, in the case of the single mode, the change of the output optical power and the output wavelength is severe due to mode hopping. Thus, by controlling both the change of the reflection spectrum and the external resonance modes as in the present embodiment, this mode hopping is suppressed. Therefore, it is possible to easily implement a single mode external resonator laser whose output optical power and output wavelength are independent of temperature change.

On the other hand, in the multi-mode, since the output optical power is almost constant regardless of the temperature change, by inserting the optical path compensation body, it is possible to implement an external resonance laser having the output optical power and the output wavelength independent of the temperature change.

The external resonance lasers of the first and second embodiments described with reference to FIGS. 4 and 6 use a structure using Bragg grating as a reflection filter to form external resonance. However, the Bragg grating of the optical fiber can be easily manufactured using a photosensitive optical fiber, but in reality, the characteristics of the reflective filter may be changed while being inserted into a ferrule and cured by using a thermosetting polymer or epoxy. . This is because stress or strain is induced in the optical fiber grating during the curing of the thermosetting polymer.

Hereinafter, an external resonance laser having an output wavelength and an output optical power independent of an external temperature as an external resonance laser employing a novel transmissive filter rather than a Bragg reflective grating of an optical fiber will be described.

7 is a cross-sectional view of an external resonance laser according to a third embodiment of the present invention, and shows the structure of the external resonance laser whose output optical power is independent of the external temperature change.

Referring to FIG. 7, the external resonance laser is a filter for selecting an oscillation wavelength and inserts a thin-film multi-layer (TFML) transmission filter 900 on the optical path instead of the Bragg grating. Since it is a transmissive filter, a separate reflector is required. To this end, an external resonance is formed by coating a material having an appropriate reflectivity on the optical fiber cross section 370.

For example, if the optical fiber end surface 370 is not coated, the reflectivity is about 3 to 5%, and when the metal film of chromium, gold, silver, platinum or the like having a thickness of 0.1 μm or less is single coated, it is about 20 to 50%. Up to 95% of multi-layer coating using metal oxides such as silicon oxide (SiO ₂ ) or aluminum oxide (Al ₂ O ₃ ), tantalum oxide (Ta ₂ O ₃ ) and titanium oxide (TiO ₂ ) Can increase the reflectivity. In addition, in order to reduce residual reflection in the front surface 920 and the rear surface 910 of the TFML transmissive filter 900, it may be inclined at a predetermined angle (Φ) with respect to the vertical surface of the optical path. The inclination angle Φ is preferably 1 to 3 degrees. It may also be AR coated to reduce reflections at the front 920 and back 910 of the TFML transmissive filter 900.

The TFML transmissive filter 900 may have a structure in which two kinds of SiO ₂ or Al ₂ O ₃ , Ta ₂ O ₅ , TiO _2, and the like, which have different refractive indices and thicknesses, are alternately stacked on a glass substrate. By adjusting the thickness of the TFML transmissive filter 900 and the number of stacked layers, a desired transmission bandwidth, transmission center wavelength, transmittance, and the like can be obtained.

On the other hand, the transmission wavelength gradient of the TFML transmission filter 900 according to the external temperature may be smaller than 0.01 nm / deg, preferably 0.003 nm / deg or less. When the transmission wavelength change is less than 0.003 nm / deg, the temperature stability of the oscillated wavelength is 0.3 nm or less when the external temperature changes by 100 degrees.

The external resonant laser of this embodiment maintains the reflection spectrum constant as in the first embodiment using TFML, so that the output optical power can be made independent of the external temperature change.

8 is a cross-sectional view of an external resonant laser according to a fourth embodiment of the present invention, which shows the structure of an external resonant laser having an output wavelength and an output optical power independent of an external temperature.

Referring to FIG. 8, the external resonant laser of this embodiment is similar to the third embodiment of FIG. 7 in the basic structure for making the wavelength independent of the external temperature change. However, in order to maintain the output wavelength of the external resonance laser irrespective of the external temperature change, the external resonance laser 1000 of the present embodiment further includes the optical path compensator 800 as in the second embodiment.

The optical path compensator 800 is formed on at least one of the left and right optical paths of the TFML transmissive filter 900 to compensate for the change in the length L _cavity of the optical path in which external resonance is formed according to the external temperature. AR coating is applied to the front surface 820 and the rear surface 810 of the optical path compensator 800, and the optical path compensator 800 is inclined at a constant angle θ with respect to the vertical surface of the optical path compensator, thereby reducing residual reflection. May be as described above.

In addition, the refractive index, the thermo-optic coefficient, and the optical path length of the optical path compensator 800 may be determined such that mode hopping is suppressed within a desired temperature range using Equations 2 and 3. For example, the length of the semiconductor chip 100 is 600um, the thermo-optic coefficient of the semiconductor chip 100 is 2.2x10 ^-4 / deg, the length of the focusing lens 200 is 1000um, the thickness of the TFML transmissive filter 900 is 1000um, When the thermo-optic coefficient of the TFML transmission filter 900 is 0.04x10 ^-4 / deg, and the thermo-optic coefficient of the optical path compensator 800 is -1.5x10 ^-4 / deg, the optical path of the optical path compensator 800 is The length is about 1000um. On the other hand, the thermo-optic coefficient of the material of the optical path compensator 800 is preferably -1.0x10 ^-4 / deg ~ -2.5x10 ^-4 / deg, and the optical path length of the optical path compensator 800 is 500um ~ 2000um May be enough.

The external resonant laser of this embodiment, like the second embodiment, can also be usefully applied to a single mode external resonant laser. That is, the external resonant laser of the present embodiment uses TFML and optical path compensator to suppress mode hopping in a single mode, thereby facilitating a single mode external resonant laser whose output optical power and output wavelength are independent of temperature change. Can be implemented.

So far, the present invention has been described with reference to the embodiments shown in the drawings, which are merely exemplary, and those skilled in the art may understand that various modifications and equivalent other embodiments are possible. will be. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

1A is a schematic cross-sectional view of an external resonance laser using a Bragg grating of a conventional TO-CAN package structure.

FIG. 1B is a cross-sectional view of the I-I portion of FIG. 1A;

2A to 2C are graphs illustrating a phenomenon in which mode hopping occurs when the external resonance laser oscillates in a single mode.

3 is a graph showing mode hopping with temperature.

4A and 4B are cross-sectional views of an external resonator laser according to a first embodiment of the present invention.

5A to 5D are graphs showing conditions for Bragg gratings used in the external resonance laser of the first embodiment.

6 is a cross-sectional view of an external resonance laser according to a second embodiment of the present invention.

7 is a cross-sectional view of an external resonance laser according to a third embodiment of the present invention.

8 is a cross-sectional view of an external resonance laser according to a fourth embodiment of the present invention.

<Description of main parts in the drawing>

100: semiconductor amplifier 110: rear of the semiconductor amplifier

120: front or exit surface of the semiconductor amplifier 200: focusing lens

300: optical fiber 320: core

340: Bragg grating 360: Clad

370: optical fiber cross section 380: etching portion of the clad

400: thermosetting polymer 500: ferrule

600: optical fiber structure 700: cavity

800: optical path compensation body 810: optical path compensation rear

820: Optical path compensation body front 900: TFML transmission filter

910: rear of TFML transmission filter 920: front of TFML transmission filter

1000: external resonance laser

Claims (15)

Semiconductor amplifiers; An optical fiber having a core on which a Bragg grating is formed and a clad surrounding the core; And And a thermosetting polymer that fixes the optical fiber to a ferrule and has a negative thermo-optic coefficient. The thickness of the cladding surrounding the core of the bragg grating is formed thinner than the cladding of the other portion, the temperature-dependent external resonator laser wraps the clad. According to claim 1, And the temperature-dependent external resonance laser is maintained by the thermosetting polymer regardless of temperature. The method of claim 2, The external resonance laser has a temperature independent external resonance laser, characterized in that the output optical power is independent of temperature changes. According to claim 1, The thickness of the cladding of the Bragg grating portion is 0.2 ~ 0.4 ㎛, The thermosetting polymer has a thermo-optic coefficient of -1 x 10 ^-4 / deg to -1.6 x 10 ^-4 / deg and a refractive index of 1.43 to 1.445. The method according to claim 1 or 4, The external resonance laser is a temperature independent external resonance laser, characterized in that it comprises an optical path compensation between the semiconductor amplifier and the optical fiber. The method of claim 5, The external resonance laser is a single-mode external resonance laser, And the output path of the external resonance laser is independent of temperature by making the optical path compensator independent of the external temperature of the length of the optical path in which the external resonance is formed. The method of claim 5, The thermo-optic coefficient of the optical path compensator is -1 x 10 ^-4 / deg ~ -2.5 x 10 ^-4 / deg, and the optical path length of the optical path compensator is 500-2000 μm, characterized in that External resonant laser. The method of claim 7, wherein The external resonance laser is formed of a thio-can (TO-CAN) package structure, and includes a focusing lens to improve the optical coupling efficiency of the semiconductor amplifier and the optical fiber, The optical path length of the semiconductor amplifier is 600 μm, the thermo-optic coefficient is 2.2 × 10 ⁻⁴ / deg, the optical path length of the focusing lens is 1000 μm, the length of the Bragg grating is 4000 μm, and the thermo-optics If the coefficient is 0.1x10 ^-4 / deg The thermo-optic coefficient of the optical path compensation body is -1.0 x 10 ^-4 / deg, the length of the optical path is a temperature-independent external resonator laser, characterized in that. The method of claim 5, The optical path compensator is coated with anti-reflection (AR) to the front and rear, And the front and rear sides are inclined at an angle with respect to the vertical plane of the optical path. The method of claim 9, The predetermined angle is a temperature independent external resonator laser, characterized in that 1 ~ 3 °. According to claim 1, The cross-section of the optical fiber is an antireflection coated, or tilted 8 ° in the vertical plane of the optical path, or a temperature-independent external resonator laser, characterized in that the non-reflective coating is tilted 8 ° in the vertical plane of the optical path. According to claim 1, The external resonance laser is a temperature-dependent external resonance laser, characterized in that the multi-mode external resonance laser is adjusted the reflection bandwidth of the Bragg grating or the pass bandwidth and the external resonance length of the TFML. According to claim 1, The external resonant laser includes a focusing lens to improve the optical coupling efficiency of the semiconductor amplifier and the optical fiber, The semiconductor amplifier has an anti-reflective coating formed on the front surface of the emission surface of the laser oscillation, and a high-reflection (HR) coating formed on the rear surface of the semiconductor amplifier. External resonance is formed between the Bragg grating and the rear surface of the semiconductor amplifier. The method of claim 13, The temperature-independent external resonator laser, characterized in that the exit surface of the semiconductor amplifier has a reflectivity of 1x10 ^-3 or less. The method of claim 13, The semiconductor amplifier includes an optical mode converter having a down-taper structure, Far-field angle (far-field angle) output from the exit surface is a temperature independent external resonator laser, characterized in that less than 25 degrees in the vertical and horizontal directions.

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