CN1588151A - Multiple wave length simultaneously exciting erbium blended optical fiber laser working and room temperature - Google Patents

Abstract

The invention relates to an erbium-doped fiber laser device working at room temperature and lasing simultaneously with multiple wavelengths, belonging to the technical field of lasers. The invention consists of an erbium-doped fiber amplifier, an output coupler and a comb filter to form a ring laser cavity, and is characterized in that at least a section of high nonlinear optical fiber is inserted into the ring laser cavity. The structure of the present invention comprehensively utilizes the advantages of high gain of erbium-doped fiber and high pump conversion efficiency and the characteristics of energy diffusion from high-power wavelength to low-power wavelength caused by parametric four-wave mixing effect in high nonlinear fiber, effectively suppressing The uniform broadening characteristics of erbium-doped fiber are realized, and multi-wavelength simultaneous lasing is realized at room temperature.

Description

A Simultaneous Lasing Erbium-doped Fiber Laser with Multiple Wavelengths Working at Room Temperature

technical field

The invention belongs to the field of laser technology, and in particular relates to the structure design of an erbium-doped fiber laser for multi-wavelength simultaneous lasing.

Background technique

Currently, wavelength division multiplexing (WDM) technology has become the mainstream technology for long-distance communication trunk lines and optical networks. The wavelength division multiplexing technology is to make full use of the bandwidth resources of the low-loss window of the optical fiber, and combine multiple optical signals of different wavelengths together for transmission in one optical fiber. Therefore, a multi-wavelength laser source is indispensable. The traditional multi-wavelength signal source is composed of a series of single-wavelength DFB semiconductor lasers. The technology is relatively simple. The disadvantage is that the system is huge and the cost is high. The problem is more prominent when considering the optical network with optical crossover nodes. From the perspective of simplifying the system structure, facilitating operation and maintenance, and reducing costs, especially in the occasions of system component debugging or component testing, laser light sources that can simultaneously output multiple wavelengths have great advantages. In addition, broadband multi-wavelength lasers are also widely used in the field of optical sensing.

Aiming at this application goal, people have done a lot of research. Rie Hayashi et al published an article titled "16-Wavelength 10-GHz Actively Mode-Locked Fiber Laser With Demultiplexed Outputs Anchored on the ITU-T Grid" on the Photonics Technology Letters (IEEE Photonics Technology Letters) in December 2003. The erbium-doped fiber cooled to liquid nitrogen temperature (77K) is used as the gain medium, and the feedback loop composed of a circulator and a lithium niobate external modulator is used as one end of the resonant cavity, and a waveguide array with a reflectivity of 90% is used as the resonant cavity. The other end is also used as a wavelength selector to realize the mode-locked pulse output of 16 wavelengths at the same time, the repetition rate is 10GHz, and the wavelength interval is 100GHz. A fatal problem of using erbium-doped fiber to achieve multi-wavelength output is that erbium-doped fiber is a uniformly broadened medium, and it is only possible to achieve multi-wavelength simultaneous stable oscillation at a very low temperature, which will be a big problem in practical applications.

K.Cheng, et al. published an article entitled "All-FiberRoom Temperature Multi-Frequency Laser with Ultra Low Cavity Loss" at the 2003 International Optical Communications Conference (OFC'2003), reporting the resonance in erbium-doped fiber The acousto-optic frequency shifting mechanism is introduced into the cavity, and multi-wavelength output is realized at room temperature by using a fused biconical fiber comb filter and a gain flattening filter. In the article entitled "Multiwavelength-Switchable SOA-Fiber Ring Laser Based on Polarization-Maintaining Fiber Loop Mirror and Polarization Beam Splitter" published in IEEE Photonics Technology Letters in January 2004 by Yong Wook Lee et al., reported The semiconductor amplifier (SOA) is used as the gain medium, and the polarization-maintaining fiber reflection loop mirror and the polarization beam splitter are used to form a comb filter, and 17 wavelengths are output at room temperature. In the article "CW Depolarized Multiwavelength Raman Fiber Ring Laser with over 58 channels and 50GHz Channel spacing" published by Nam Seong Kim et al. at the 2002 International Optical Communications Conference (OFC'2002), it was reported that 6 semiconductor laser pumps with 3 wavelengths were used The Raman gain obtained by Pu's 16km long dispersion compensating fiber (DCF) uses the fiber F-P tunable filter as the wavelength selection element, and realizes the output of up to 58 wavelengths in the ring cavity structure, and the wavelength interval is 50GHz. Using semiconductor SOA or Raman gain, or using the method of acousto-optic frequency shifting can overcome the problem of uniform broadening, and multi-wavelength oscillation can be realized at room temperature. However, the semiconductor SOA is not an all-fiber device, the coupling loss is too large, and the output power is limited; the Raman gain coefficient is small, and generally requires a longer fiber length and a larger pump power, which greatly improves the output characteristics. restrictions; and the use of acousto-optic frequency shifting method has more stringent requirements on the equipment and so on. On the other hand, multi-wavelength simultaneous oscillation can also be achieved by using fiber parametric gain. The parametric gain coefficient of the same fiber is slightly higher than the Raman gain coefficient, but the requirements for phase matching and pump light quality are higher than those for Raman gain. It is still in the research stage. In conclusion, so far, the technology of simultaneously realizing multi-wavelength output lasers has not been able to meet the needs of applications.

Contents of the invention

In order to overcome the deficiencies of the prior art, the present invention proposes a ring cavity erbium-doped fiber laser with a new structure. This structure introduces a mechanism of parametric four-wave mixing in the ring cavity, and this mechanism can suppress the erbium-doped fiber laser. The uniform broadening characteristics can achieve multi-wavelength simultaneous lasing at room temperature. The invention only needs to introduce a section of passive high nonlinear optical fiber, has simple structure and is easy to realize.

The multi-wavelength simultaneous lasing erbium-doped fiber laser proposed by the present invention is composed of an erbium-doped fiber amplifier, an output coupler and a comb filter to form a ring laser cavity, which is characterized in that in the ring laser cavity Insert at least one section of highly nonlinear fiber.

The erbium-doped fiber amplifier currently has a variety of commercial products to choose from, such as C-band amplifiers, L-band amplifiers or C+L-band amplifiers, which can be selected according to the wavelength range required by the application target of the multi-wavelength laser.

The high nonlinear optical fiber is currently available as a commercial product. The zero dispersion wavelength of highly nonlinear fiber should be selected near the gain wavelength range of erbium-doped fiber.

The comb filter can be realized by various methods such as melting biconical fiber, birefringent fiber loop mirror, and Fabry-Perot cavity. It is a relatively mature technology in the field of optoelectronics and has been widely used in various multi-wavelength lasers. use. Commercial products are currently available.

The output coupler is a conventional product.

Working principle of the present invention:

The basic structure of the room temperature multi-wavelength erbium-doped fiber laser proposed by the present invention is shown in FIG. 1 . This is a ring cavity structure, which is composed of an erbium-doped fiber amplifier 11, an output coupler 12, a highly nonlinear optical fiber 14 and a comb filter 13 connected one by one. The difference from the ordinary ring cavity erbium-doped fiber laser is that a section of highly nonlinear fiber and a comb filter are inserted, and the comb filter has been commonly used in multi-wavelength lasers realized by other methods. Erbium-doped fiber amplifiers contain optical isolators so that the light in the cavity can only travel in one direction. According to the basic principle of the laser, in an ordinary ring cavity erbium-doped fiber laser (that is, the highly nonlinear fiber and comb filter are removed from the structure shown in Figure 1), the laser oscillation first occurs in the cavity with the smallest loss, generally an erbium-doped fiber amplifier generated at the wavelength of maximum gain. In practice, even if the erbium-doped fiber amplifier in the ring cavity adopts gain flattening filtering measures, the gain at each wavelength cannot be exactly the same, and the laser oscillation will first start to oscillate at the wavelength with the highest gain, generally multi-mode oscillation . If there are several maxima with relatively close sizes in the gain spectrum of the erbium-doped fiber, the oscillation may occur simultaneously at these maxima, that is, several wavelengths may oscillate at the same time. However, due to the uniform broadening characteristics of the erbium-doped fiber, there is competition between the modes of oscillation, which are very unstable.

When a comb filter and a highly nonlinear optical fiber are added to the ring cavity, on the one hand, the oscillation wavelength is not only affected by the maximum gain of the erbium-doped fiber amplifier, but also affected by the filter transmission peak. Oscillation can only occur at wavelengths where the gain is large and at the peak of the filter, but this does not change the multi-mode oscillation and the competition between modes. On the other hand, parametric four-wave mixing occurs after light of different wavelengths enters a highly nonlinear fiber. Gain is lost while the originally high-power wavelength is lost (negative gain). If the wavelength of the gain falls within the transmission peak of the comb filter, it can enter the erbium-doped fiber again and obtain the gain of the erbium-doped fiber like the input signal light. This gain process consumes the upper energy level in the erbium-doped fiber The number of particles has a dampening effect on the earliest occurring oscillations. After many cycles, the power of each wavelength in the cavity reaches a balance, and all those wavelengths with sufficient gain to overcome the loss can maintain oscillation. Therefore, the uniform broadening characteristics of erbium-doped fibers are generally suppressed, and simultaneous stable lasing at multiple wavelengths at room temperature can be achieved.

The technical indicators for measuring multi-wavelength lasers include: number of output wavelengths, covered wavelength range (bandwidth), power difference (fluctuation degree) of each wavelength, output power, extinction ratio and wavelength interval, spectral line width of each wavelength, etc. The key technical indicators are the wavelength range covered, the degree of fluctuation and the extinction ratio. Theoretical and experimental research results prove that the covered wavelength range mainly depends on the flat gain bandwidth of the erbium-doped fiber amplifier. The wider the flat gain bandwidth of the amplifier, the larger the possible covered wavelength range. The extinction ratio is related to the contrast of the comb filter, but mainly depends on the amplified spontaneous emission (ASE) floor of the output of the Erbium-doped fiber amplifier. In addition to being related to the flat gain bandwidth of the amplifier, the degree of fluctuation mainly depends on the parametric four-wave mixing gain obtained by the light of each wavelength in the highly nonlinear fiber, because this four-wave mixing is to achieve multi-wavelength stable oscillation. The essential. Theoretically, under logarithmic coordinates, the degree of fluctuation has a negative linear relationship with the product of the average power density of the input high nonlinear fiber, the nonlinear coefficient of the high nonlinear fiber and the effective length. The following examples will prove this a little.

Therefore, when the wavelength range covered by the multi-wavelength output is required to be large and the fluctuation is as small as possible, the selected erbium-doped fiber amplifier should have a flat gain spectrum to ensure a wide flat gain bandwidth, on the premise that it is sufficient to overcome the cavity loss The lower lock-in gain should be as low as possible with high output power. A low gain can make the ASE substrate low and the extinction ratio of the output multi-wavelength laser is high, and the high power is beneficial to reduce the degree of fluctuation and increase the output power. In addition to requiring the zero dispersion wavelength to be near the gain wavelength of the erbium-doped fiber, the highly nonlinear fiber should also require a high nonlinear coefficient, small dispersion, and a flat dispersion slope as possible. At the same time, the length of the highly nonlinear optical fiber can be appropriately lengthened without making the total loss of the ring cavity exceed the gain of the amplifier. The high nonlinear optical fiber of existing products is generally about 500m to 2000m in order to generate a sufficiently strong parametric four-wave mixing frequency. The wavelength interval of the output multi-wavelength laser depends on the wavelength interval of the comb filter. After the coverage wavelength range and wavelength interval are determined, the number of output wavelengths is determined. The spectral line width of the multi-wavelength output has a great relationship with the width of each "tooth" of the comb filter, and the spectral line width of the laser output from the oscillation is generally smaller than the broadband of the "tooth". Therefore, parameters such as the wavelength range, wavelength interval, and tooth width of the comb filter can be determined according to application requirements.

For the purpose of achieving multi-wavelength stable oscillation, the erbium-doped fiber amplifier, output coupler, highly nonlinear fiber and comb filter in the ring cavity can be connected in any order. However, when the output power ratio is relatively high, if the output coupler is placed at the input end of the high nonlinear fiber, it will affect the power density input to the high nonlinear fiber; and when the power density is relatively high, the high nonlinear fiber The four-wave mixing process in the linear fiber will broaden the spectral line of a single wavelength, so if the output coupler is placed at the output end of the highly nonlinear fiber, it is possible to obtain a slightly broadened spectral line width. These nuances can be chosen or compromised depending on the requirements of the application.

Features and effects of the present invention:

The structure of the present invention comprehensively utilizes the advantages of high gain of erbium-doped fiber and high pump conversion efficiency and the characteristics of energy diffusion from high-power wavelength to low-power wavelength caused by parametric four-wave mixing effect in high nonlinear fiber, effectively suppressing The uniform broadening characteristics of erbium-doped fiber are realized, and multi-wavelength simultaneous lasing is realized at room temperature.

Description of drawings

Fig. 1 is the principle structural representation of the multi-wavelength erbium-doped fiber laser that the present invention proposes;

Fig. 2 is embodiment one of the present invention, the figure of C-band broadband multi-wavelength erbium-doped fiber laser experiment apparatus;

Fig. 3 is embodiment one of the present invention, the C-band broadband multi-wavelength output laser spectrum measured at room temperature;

Fig. 4 is the relationship between the output laser spectral fluctuation degree measured for the device shown in Fig. 2 and the total input power of the high nonlinear fiber, the nonlinear coefficient of the high nonlinear fiber and the product of the nonlinear effective length;

Fig. 5 is the second embodiment of the present invention, C+L band broadband multi-wavelength erbium-doped fiber laser experimental device diagram;

Fig. 6 is the C+L band multi-wavelength output laser spectrum measured at room temperature in Example 2 of the present invention.

Detailed ways

The erbium-doped fiber laser of the room temperature broadband multi-wavelength simultaneous lasing proposed by the present invention is described in detail in conjunction with the embodiments and accompanying drawings as follows:

Embodiment 1, the structure of an erbium-doped fiber laser with multiple wavelengths simultaneously lasing at room temperature is shown in FIG. 2 . An erbium-doped fiber amplifier 21, a variable optical attenuator 251, a directional coupler 22 with a coupling ratio of 10:90, a highly nonlinear optical fiber 24, a comb filter 23 and a variable optical attenuator 252 are sequentially connected to form a ring laser cavity. Wherein, the common end 221 of the directional coupler is connected to the variable optical attenuator 251, the 90% end 223 is connected to the high nonlinear optical fiber 24, and the 10% end 222 is the output end of the ring cavity. The erbium-doped fiber amplifier has the function of automatic flat locking of the gain spectrum, the gain wavelength covers the C-band range, the flat gain is 24dB, and the maximum output power is 18dBm. The high nonlinear fiber is 1450 meters long, the zero dispersion wavelength is at 1548nm, the dispersion slope at 1550nm is 0.0155ps/nm ² ·km, the nonlinear coefficient is 11.9W ^-1 km ^-1 , and the loss coefficient is 0.73dB/km. The comb filter is a Sagnac interference ring filter made of high birefringence fiber, and the interval period of the filter peak is 0.31nm. A directional coupler with a coupling ratio of 10:90 allows 10% of the light in the cavity to be coupled out, and the remaining 90% of the light is fed back into the cavity. Variable optical attenuator 251 is used to adjust the amount of power entering the highly nonlinear fiber, while variable optical attenuator 252 is used to adjust the overall loss of the cavity so that the power level oscillating back to the erbium-doped fiber amplifier remains at the locked input of the amplifier Within the range, in order to obtain multi-wavelength laser output with relatively equal power of each wavelength. In actual use, this adjustment can be realized by changing the coupling ratio of the directional coupler 22, the gain level of the erbium-doped fiber amplifier, and the input dynamic range. At this time, 251 and 252 can be canceled.

In this embodiment, the output spectrum measured by the spectrometer at the output end is shown in FIG. 3 . There are 81 wavelength laser outputs in the wavelength range from 1539.28nm to 1564.02nm, and the wavelength interval is 0.31nm, which is determined by the wavelength interval of the comb filter used. The inset of Figure 3 is enlarged to show a few of the 81 output wavelengths. It can be seen that the output spectral lines of each wavelength are very regular and actually very stable. It can also be seen from the illustration that the extinction ratio of the output laser line is close to 30dB, which is obviously greater than the contrast ratio of about 20dB of the comb filter. In the output spectrum shown in Figure 3, the output power level of each wavelength has certain differences. The power is relatively high near the wavelength of 1543nm and 1555nm, and the power is about 7dB lower near the wavelength of 1547nm. This is caused by the slightly higher gain of the erbium-doped fiber amplifier near 1543nm and 1555nm, where the laser oscillation first starts. Figure 4 shows the relationship between the power fluctuation degree of the output spectrum of the multi-wavelength laser and the total input power of the high nonlinear fiber, the nonlinear coefficient of the high nonlinear fiber and the nonlinear effective length product P _tot γL _eff based on the experimental data , where the gain fluctuation is defined as the power difference between the maximum value of the output power and the minimum value in the middle depression, expressed in dB. The curve in Fig. 4 shows that increasing the total input power of the high nonlinear fiber, the nonlinear coefficient or the length of the high nonlinear fiber is beneficial to reduce the power fluctuation of each wavelength in the output spectrum. It should be noted that during the measurement of the experimental data shown in Figure 4, the covered wavelength range has not changed. If the total input power is divided by the covered wavelength range, the degree of fluctuation and the average power density have a similar negative linearity. relation. In addition, the nonlinear effective length is related to the actual length of the optical fiber and the loss coefficient. For an optical fiber of the same length, the greater the loss, the shorter the effective length.

Embodiment 2, the structure of an erbium-doped fiber laser with C+L band broadband multi-wavelength simultaneous lasing at room temperature is shown in FIG. 5 . The difference from the structure shown in FIG. 2 is that the erbium reference fiber amplifier used in this embodiment is composed of two branches of C-band amplification and L-band amplification connected in parallel. Wherein, the C-band branch is composed of a C-band erbium-doped fiber amplifier 511 and a variable optical attenuator 551 connected to its input end, and the L-band branch is composed of a long-wave loss device 56, a variable optical attenuator 552 and an L-band Erbium-doped fiber amplifiers 512 are connected in sequence. The two branches are connected in parallel by two C+L wavelength division multiplexing couplers 521 and 522 to form a C+L broadband amplification combination. C+L broadband amplification combination, high nonlinear fiber 54, comb filter 53 and 10:90 directional coupler 523 are connected in sequence to form a ring laser cavity. Terminals 5212 and 5213 of the C+L wavelength division multiplexing coupler 521 are connected to the variable optical attenuator 551 and the long-wave loss device 56 respectively. Terminal 5211 is connected to 5233 (90%) of 10:90 directional coupler 523 . Ports 5222 and 5223 of the C+L wavelength division multiplexing coupler 522 are connected to the output port of the C-band Erbium-doped fiber amplifier 511 and the output port of the L-band Erbium-doped fiber amplifier 512, respectively. The 5221 end is connected with the high nonlinear optical fiber 54. The comb filter 53 is connected to the common terminal 5231 of the 10:90 directional coupler 523, and 5232 (10%) is the output terminal of the ring cavity.

Both the C-band and L-band erbium-doped fiber amplifiers have the function of automatically flattening the gain spectrum, the flat gain is 24dB, and the maximum output power of each is 18dBm. The long-wave loss device 56 is a device made of bent optical fiber, and the loss to light increases as the wavelength increases. The high nonlinear fiber used is the same as that used in Figure 2. The comb filter is a product filter (Micron Optics, Inc) based on F-P cavity, covering both C and L bands, with a wavelength interval of 0.8nm, an insertion loss of 2dB, and a contrast ratio of 28dB. The variable optical attenuators 551 and 552 are used to adjust the power levels entering the C-band and L-band erbium-doped fiber amplifiers respectively, so as to obtain a multi-wavelength output laser spectrum as flat as possible.

If the output power of the Erbium-doped fiber amplifier is large enough, the long-wave loss device 56 is not necessary. In addition, the losses of the variable optical attenuators 551 and 552 can also be reduced or even eliminated. However, under the experimental conditions of the present embodiment, the maximum output power of erbium-doped fiber amplifiers 511 and 512 is limited, and the output spectrum of 512 is quite flat so that up to 4 wavelengths can start to oscillate simultaneously. The power spectral density of the linear optical fiber 54 is too low to achieve the required parametric four-wave mixing effect, that is, the power difference of each wavelength in the output multi-wavelength laser spectrum is relatively large. The introduction of the long-wave attenuator 56 and the proper adjustment of the variable optical attenuator 552 make the partial oscillation of the longest wavelength in the L-band suppressed while the partial power of the shorter wavelength is relatively concentrated, and finally a relatively flat spectral output is obtained.

FIG. 6 shows the output spectrum of C+L band broadband multi-wavelength simultaneous oscillation measured in the second embodiment. The output spectrum has 30 wavelengths in the range from 1539.53nm to 1562.79nm, and 20 wavelengths in the range from 1567.69nm to 1583.44nm, with a wavelength interval of 0.8nm. The -10dB linewidth of a single wavelength in the output spectrum measured by a spectrometer at a resolution of 0.01nm is 0.0432nm, which is obviously narrower than the original linewidth (0.0844nm) of the comb filter. The extinction ratio of the output spectrum is greater than 33dB, which is also higher than the contrast (28dB) of the comb filter, indicating that the output is indeed an oscillating laser output.

Claims (1)

1, a kind of erbium doped fiber laser of multi-wavelength simultaneous lasing of working and room temperature, connect and compose the loop laser chamber by Erbium-Doped Fiber Amplifier (EDFA), output coupler and comb filter, it is characterized in that, in this loop laser chamber, insert one section highly nonlinear optical fiber at least.

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