KR100775413B1 - Single mode light source device having external cavity - Google Patents

Abstract

The present invention relates to a single mode light source device having an external resonator. The single mode light source device according to the present invention includes a laser diode, a condenser lens, and an optical fiber having an incident surface cut at a predetermined angle to reflect part of the light output from the laser diode and return it to the laser diode. The internal resonator of the laser diode itself and the external resonator formed between the light emitting surface of the laser diode which performs multi-mode oscillation and the incident surface of the optical fiber are combined to select and output only a single mode (wavelength) optical signal. According to the present invention, it is possible to easily and inexpensively implement a single mode light source device, which can be used as a variable wavelength light source in a wavelength division multiple optical communication network.

Single mode light source, laser diode, external resonator, wavelength multiplexing, optical communication network, wavelength variable light source

Description

Single mode light source device having external cavity

1A is a structural diagram of a single mode light source device according to an embodiment of the present invention.

1B illustrates another embodiment of forming an external resonator.

2 is a diagram illustrating a principle of selecting a single mode using an external resonator in a single mode light source device according to an exemplary embodiment of the present invention.

3 is a diagram illustrating an example of a spectrum of a conventional multi-mode light source and a spectrum of a single-mode light source device according to an embodiment of the present invention.

4A and 4B illustrate examples of modulation characteristics of a single mode light source device according to an exemplary embodiment of the present invention.

5 is a diagram illustrating an example of a variable wavelength performance of a single mode light source device according to an exemplary embodiment of the present invention.

The present invention relates to a light source device, and more particularly, to a single mode (wavelength) light source device having an external resonator that can be used in a wavelength division multiplexing network (WDM network) or the like.

As the demand for information communication and various services increases, various analog and digital optical communication systems have been utilized.In the early days of optical communication, data transmission has been increased through time division multiplex (TDM), but the information capacity and electronic devices have been increased. Due to the limitations of the present invention, wavelength division multiplex (WDM) technology, which can divide and use wavelength components of an optical signal, is incorporated.

Wavelength Division Multiplexing (WDM) is a method of multiplexing multiple wavelengths of light and then splitting each wavelength backwards at the receiving end. Its main components consist of an optical transmitter, optical receiver, wavelength multiplexer, wavelength demultiplexer, optical amplifier, optical fiber, and optical signal compensator. Of these configurations, the key module and the most expensive part is the optical transmitter.

As a light source used in an optical transmitter, a distributed feedback laser diode (DFB-LD) is mainly used. This has the advantage of having a high Side Mode Suppression Ratio (SMSR), high power, and narrow linewidth, but has the disadvantage of being expensive. Therefore, in the wavelength division multiplexing optical subscriber network where price is very important, it is difficult to apply DFB-LD in reality.

In the current optical subscriber network, a scheme in which each user divides bandwidth over time using a time division multiplexer / time division multiple access (TDM / TDMA) scheme is commonly used. However, since this method uses time-division, the transmission speed decreases when the number of connected subscribers increases, and the implementation of a protocol for locating each subscriber data without collision with other subscriber data in time becomes complicated. Thus, many researchers continue to attempt to implement cheap WDM laser light sources and apply them to WDM-PON.

The first method of such an attempt is to split an incoherent light source of a wide band in the wavelength region. This method mainly uses LEDs, single mode laser diodes (SLDs), and optical amplifiers. LED is difficult to use because the output optical power is weak, SLD is expensive, and the optical amplifier has a problem that requires an expensive external modulator for generating an optical signal.

The second method injects an external beam into a low-cost Febry Perot laser and induces a wavelength-locked phenomenon that reflects only the mode corresponding to the selected wavelength band. ), A Fabry Perot laser is used as the WDM light source. Coherent or non-coherent beams may be injected into the external beams.

However, both methods have a disadvantage in that an external light source for generating an external beam to be injected is expensive. In addition, according to the above-described method, since the structure is not only complicated but also interlocked with the external beam, the reliability of the operation is not guaranteed.

The third method is to induce wavelength immersion using a magnetic beam of a Fabry Perot laser. This is a method of injecting a magnetic beam into an optical fiber Fabry Perot laser using an optical fiber Bragg grating filter, which is difficult to realize at a low cost and difficult to directly modulate the laser beam.

Another method is to induce wavelength immersion with a beam reflected from an optical fiber grating filter outside a Fabry Perot laser diode. This method can be limited to a method of gain switching driving a Fabry-Perot laser diode to generate an optical pulse train, and has a disadvantage in that it is difficult to produce a high performance optical pulse due to optical interference between magnetic wavelengths.

As such, the Fabry-Perot laser diode has a simple structure and a low cost, but is not suitable as a light source of a wavelength division multiple optical network because it is a multi-mode laser that oscillates several wavelengths simultaneously. In addition, even when only a single mode is selected using an optical wavelength filter, a stable output cannot be generated by mode hopping, and thus it cannot be used in a wavelength division multiplexing optical network.

Therefore, an object of the present invention is to provide an optimal single mode light source device that can be used in a wavelength division multiple optical communication network.

In addition, another technical problem to be achieved by the present invention is to provide a single mode light source device that can be implemented at low cost while operating stably.

Another object of the present invention is to provide a single mode light source device that provides a single mode of light by using a multi-mode laser diode.

According to an aspect of the present invention, there is provided a single mode light source device, including: a laser diode including an internal resonator for generating one or more optical signals having one or more wavelengths; A lens for condensing the optical signal; And a transmission optical fiber for transmitting the optical signal collected from the lens. In addition, by inclining the incident surface to which the optical signal is incident on the optical fiber at a predetermined angle, an external resonator structure is formed between the incident surface and the light emitting surface of the laser diode, so that the light source device is in a single mode. Rash.

The inclination angle of the incident surface of the optical fiber is adjusted so that the optical signal reflected from the incident surface can reach the laser diode and be resonated. In this case, when the light reflected from the incident surface of the optical fiber is incident to the laser diode, it is set to a predetermined length of the external resonator so that phase alignment occurs well to prevent generation of noise including interference and unstable output. Can be.

According to one aspect of the invention, the laser diode including an internal resonator for generating one or more optical signals having one or more wavelengths; A first lens and a second lens for condensing the optical signal; A transmission optical fiber for transmitting the optical signal focused by the first lens and the second lens; And a partial reflection mirror positioned between the first lens and the second lens 60, wherein an external resonator structure is formed between one surface of the partial reflection mirror and the light emitting surface of the laser diode to oscillate in a single mode. A single mode light source device is provided.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.

In an embodiment of the present invention, a multi-mode laser diode is used to achieve stable single mode oscillation without mode hopping. To this end, by adjusting the inclination angle of the incidence plane of the optical fiber coupled to the currently commercially available multi-mode laser diode (for example, Fabry Perot laser diode) by using the gap structure between the laser diode and the optical fiber as a feedback resonator, The present invention provides a light source device having a new structure that oscillates only a stable single mode among various modes generated in a diode.

Next, a detailed structure of the light source device according to the embodiment of the present invention will be described.

1A is a structural diagram of a single mode light source device according to an embodiment of the present invention.

A single mode light source device according to an embodiment of the present invention, as shown in the accompanying Figure 1a, a laser diode 10 for generating light, a light collecting lens 20 for condensing the light generated from the laser diode, and the lens It includes an optical fiber 30 for transmitting the light collected through.

The laser diode 10 is a multi-mode laser diode that simultaneously generates light having several wavelengths. For example, a laser diode chip having a Fabry Perot etalon structure may be used. Here, the laser diode chip having the above structure is used, but is not necessarily limited thereto. At different wavelengths produced in the laser diode, the spacing between each wavelength can be determined by the length of the chip. Light emitted from such a laser diode may spread at an angle.

The condenser lens 20 collects light emitted from the laser diode 10 and transmits the light to the optical fiber 30. The light collecting lens 20 may be an aspherical or spherical lens. However, since the reflectance of the cross section of the optical fiber which is not specially processed is very small, about 15dB, the coupling efficiency by the lens is better to increase the role of the external resonator, and therefore, it is advantageous to use an aspherical lens.

The optical fiber 30 transmits the light collected by the condenser lens 20 to an external device (not shown). In particular, the surface on which the light is incident is cut and tilted at a predetermined angle. .

Next, an operation of the single mode light source device having the above structure will be described.

In the laser diode 10, one or more optical signals having multiple modes, that is, one or more wavelengths, are generated, and the optical signals are transmitted to the optical fiber 30 by the condenser lens 20. At this time, some of the optical signals (for example, about 4%) that are collected by the condenser lens 20 and reach the optical fiber 30 are reflected by the medium refractive index difference (the difference in refractive index between the air and the optical fiber).

At this time, if the incident surface of the optical fiber is tilted at a predetermined angle, that is, cut at a predetermined angle, the optical signal reflected from the incident surface of the optical fiber can be prevented from entering the laser diode again. When the incident surface of the optical fiber 30 is cut at a predetermined angle, for example, 6 ° to 8 °, the reflected optical signal does not enter the laser diode 10. In this case, if the angle is reduced or even made zero, most of the reflected light enters the laser diode again. This creates the effect that a resonator is formed between the light emitting surface on which light is output from the laser diode 10 and the incident surface of the optical fiber 30. Therefore, in the embodiment of the present invention, the inclination angle of the incident surface of the optical fiber is set so that the light reflected from the incident surface can reach the laser diode and resonate. The tilting angle of the incidence surface to form such a resonance may be referred to as a "set angle", and the resonator thus formed is referred to as an "external resonator". The set angle is preferably such that light reflected from the optical fiber cross section can be incident to the laser diode as much as possible in consideration of the fixed position and angle of the laser diode.

The external resonator is combined with a self resonator of the laser diode 10 (hereinafter referred to as an "internal resonator") to induce oscillation of a specific wavelength under certain conditions. That is, the optical signal of a specific wavelength is selected by the internal resonator of the laser diode of the Fabry-Perot etaloon structure and the external resonator formed between the laser diode 10 and the optical fiber 30.

1B illustrates another embodiment of forming an external resonator. Instead of using the laser cross section and the optical fiber cross section as the external resonator, the partial reflection mirror 50 inserted separately from the laser cross section is used as the external resonator. At this time, the insertion space of the partial reflection mirror 50 is provided and two lenses are used to focus the light for an effective feedback effect of the partial reflection mirror 50. In addition, unlike the case of Figure 1a, since the light beam reflected from the optical fiber cross-section does not cause a feedback effect to enter the laser diode, the optical fiber cross-section has an appropriate angle so that the reflected light exits in the other direction. The partial reflection mirror 50 refers to a mirror that passes some light and reflects some light, and the transmission ratio and the reflection ratio are determined in consideration of the formation conditions of the external resonator and the final output power of the light beam. One side of the partially reflective mirror is also anti-reflecting to prevent the formation of another resonator inside the partially reflective mirror Lens 1 (40) is used to direct light from the laser diode to lens 2 (60). Through condensing into the optical fiber 30 and at the same time it has to play a role of condensing the light reflected from the partial reflection mirror 50 back to the laser diode (10).

2 illustrates a principle in which a single wavelength optical signal is selected by a combination of an external resonator and an internal resonator in a single mode light source device according to an exemplary embodiment of the present invention.

2 (a) shows the gain distribution of the laser diode, and only the wavelength whose gain exceeds the loss among the selected wavelengths in the internal resonator is oscillated. FIG. 2B illustrates a wavelength selected by the internal resonator of the laser diode, and FIG. 2C illustrates a wavelength selected by the external resonator between the light emitting surface and the optical fiber cutting surface of the laser diode.

The spacing between the wavelengths selected by each resonator is inversely proportional to the refractive index of the medium and the length of the resonator.

Δλ represents the interval between wavelengths, λ represents the wavelength of the mode, L represents the length of the resonator, and n represents the refractive index of the medium forming the resonator.

In an embodiment of the present invention, since the length of the external resonator is larger than the length of the internal resonator, wavelengths selected from the external resonator according to Equation 1 above have a smaller interval. Oscillation in the laser diode 10 occurs at a wavelength in which (b) of FIG. 2 and (c) of FIG. 2 coincide among the wavelengths falling into the gain distribution of (a) of FIG. 2, and (d) of FIG. ). That is, among the wavelengths selected in the internal resonator of the laser diode and those selected in the external resonator between the light emitting surface and the optical fiber cutting surface of the laser diode, only the same wavelength is selected.

FIG. 3 (a) shows the oscillation spectrum of a general Febri Ferret laser diode, and FIG. 3 (b) shows the oscillation spectrum of a light source device having an external resonator according to an exemplary embodiment of the present invention. Referring to FIG. 3, in the conventional laser diode, a plurality of wavelengths are selected, whereas in the light source device according to the exemplary embodiment of the present invention, one wavelength is selected as shown in FIG. 3B and the other wavelengths are easily suppressed. have.

The experiment was conducted to prove that the selected wavelength is stably output.

4A is a diagram illustrating an experimental device for measuring characteristics of a light source device according to an exemplary embodiment of the present invention, and FIG. 4B illustrates the results measured by the experimental device shown in FIG. 4A.

As shown in FIG. 4A, the optical filter 200 is connected to the light source device 100 according to the exemplary embodiment, and the oscilloscope 300 is connected to the optical filter 200 to perform a modulation experiment. The output of the light source device 100 according to the embodiment of the present invention having an external resonator is transmitted to the optical filter 200 and passed therethrough. At this time, the pass band wavelength of the optical filter 200 is matched with a specific wavelength selected by the combination of the external resonator of the light source device and the internal resonator of the laser diode according to the embodiment of the present invention. As a result of observing the optical signal passing through the optical filter 200 with the oscilloscope 300, the result as shown in FIG. 4B was obtained. The modulation pattern is a pseduo random bit sequence (PRBS), and it can be seen that the oscilloscope observation results are very clean.

Therefore, in the conventional Fabry-Perot laser diode, when a random wavelength is selected as an optical filter, a clean waveform cannot be obtained due to mode partition noise, but in a light source device having an external resonator according to an embodiment of the present invention, a specific wavelength is used. It can be seen that it is possible to select only the optical signal of.

On the other hand, the oscillation wavelength of the light source device according to an embodiment of the present invention can be adjusted by the value of the temperature and the bias current. The oscillation wavelength is determined at the point where the resonance condition of the internal resonator of the laser diode (the laser diode of the Fabry-Perot etaloon structure) coincides with the resonance condition of the external resonator.

The resonance condition of the external resonator formed between the light emitting surface of the laser diode and the incident surface of the optical fiber does not change if the positions of the laser diode and the optical fiber are fixed. Therefore, the length of the external resonator (the distance between the light emitting surface of the laser diode and the incident surface of the optical fiber) is such that the occurrence of unstable output and noise, including interference, when light reflected from the optical fiber incident surface is reflected by the laser diode and is incident thereon. In order to prevent this from happening, it is preferable to set the length of the phase alignment well. In addition, when the length of the external resonator becomes an integer multiple of the length of the internal resonator, multiple wavelengths in which phase alignment occurs may occur at the same time.

However, the semiconductor laser diode may change the length of the internal resonator in accordance with the temperature and the bias current or change the length of the internal resonator due to the change in the refractive index. Thus, the wavelength selection condition may be adjusted by adjusting the resonance condition of the internal resonator. That is, the resonance condition of the external resonator as shown in (c) of FIG. 2 is fixed. Since the resonant condition of the internal resonator of FIG. 2B can be changed by the change of temperature and current, the wavelength selection condition determined by the combination of the external resonator and the internal resonator is varied, resulting in the optical signal selected and outputted. The wavelength of can be varied.

As such, if the specific wavelength is satisfied, the wavelength can be varied within the gain distribution of the laser diode. Therefore, the light source device according to the exemplary embodiment of the present invention may be used as a variable wavelength laser light source. 5 is a light source device according to an embodiment of the present invention, when using the same laser diode, when the oscillation wavelength of the light source device according to the embodiment of the present invention is changed when the temperature and the current applied to the laser diode is changed It is an example showing that.

The structure and embodiment of the invention described above are described by way of example and not by way of limitation. Any person with ordinary skill in the art can make various imitations and modifications without departing from the basic principles of the invention.

As such, according to an embodiment of the present disclosure, a single mode light source device that can be stably operated and can be implemented at low cost can be provided.

In particular, the conventional laser diode uses a separate mirror or optical fiber grating to implement an external resonator as described above, but in the embodiment of the present invention, the incident surface of the optical fiber to which the light output from the laser diode is coupled is cut at a predetermined angle. Since it is used as a reflective surface, it is possible to implement a light source device simply and cheaply using an existing packaging structure without any means.

The single mode light source device according to the embodiment of the present invention may be used as a light source of a wavelength division multiple optical communication network, and may also be used as a variable wavelength light source. In particular, it can be utilized as a low cost light source for the WDM-PON, especially among the wavelength division multiple optical communication networks.

Claims (7)

A laser diode comprising an internal resonator for generating one or more optical signals having one or more wavelengths; A lens for condensing the optical signal; And Transmission optical fiber for transmitting the optical signal collected from the lens Including, The incidence plane to which the optical signal is incident in the optical fiber is tilted at a predetermined angle, thereby forming an external resonator structure between the incidence plane and the light emitting surface of the laser diode to oscillate in a single mode. And the incident surface is tilted at an angle at which light reflected from the incident surface is incident to the laser diode. A laser diode comprising an internal resonator for generating one or more optical signals having one or more wavelengths; A first lens and a second lens for condensing the optical signal; A transmission optical fiber for transmitting the optical signal focused by the first lens and the second lens; And A partially reflective mirror positioned between the first lens and the second lens 60 Including, An external resonator structure is formed between one surface of the partially reflective mirror and the light emitting surface of the laser diode to oscillate in a single mode, And one side of the partially reflective mirror has an anti-reflecting coating, and the other side transmits a part of the optical signal and reflects the other part to be incident to the laser diode. delete delete delete The method according to any one of claims 1 and 2, And the laser diode is a Fabry-Perot laser diode. The method according to any one of claims 1 and 2, And the laser diode is a laser diode in which the resonance condition of the internal resonator is changed by a current applied to the laser diode or a temperature change of the laser diode.

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