KR20060020165A - Tunable optical signal generator - Google Patents

Abstract

The present invention relates to an optical oscillation device, comprising: an electrical signal generation device for generating a predetermined electrical signal; The laser is driven by the electrical signal to generate an optical signal of the first frequency and the optical signal of the first sideband modes spaced by the modulation frequency interval of the electrical signal around the first frequency (hereinafter referred to as a master laser) Called); And

The optical signal is driven by the electrical signal to generate an optical signal of a second frequency, and generates an optical signal of second sideband modes spaced apart by a modulation frequency interval of the electrical signal about the second frequency, and outputs from the master laser. And a laser (hereinafter referred to as a slave laser) for receiving the received optical signal and mixing and outputting the received optical signal with the optical signal of the second frequency.

According to the present invention, it is possible to generate an optical signal having a frequency in a wide range from tens of MHz to several THz as an ultrafast optical signal generation technique through high speed modulation. In addition, based on this, it is possible to implement an optical device such as a semiconductor laser or an optical modulator required in a high speed optical communication system.

Ultra high frequency, optical signal generator, microwave, optical millimeter wave fusion, laser

Description

Frequency variable optical signal generator {Tunable optical signal generator}

1 is a block diagram of a frequency variable type optical oscillation apparatus according to the present invention.

FIG. 2 (a) shows the continuous wave output from the master laser and the slave laser of FIG. 1 and the peak of the FWM conjugate when these two continuous waves are interacted with each other.

2 (b) and 2 (c) show the spectrums of the optical signals outputted from each other when an electrically RF modulated signal is applied to the master laser and the slave laser.

FIG. 2 (d) shows the red shift of the lasing frequency f _SL of the slave laser according to the injection locking.

3 (a) to 3 (f) show the results of measuring the output of the slave laser by the optical spectrum analyzer when the light output from the master laser is injected into the slave laser according to the present invention.

4 (a) to 4 (f) show the results of measuring the output of the slave laser by an RF spectrum analyzer when the light output from the master laser is injected into the slave laser according to the present invention.

5 illustrates the relationship between the bit signal frequency and the power of the modulated signal.

TECHNICAL FIELD The present invention relates to an optical oscillation device, and more particularly, to an apparatus for oscillating light of microwaves or millimeter waves.

As access to and use of the wireless Internet capable of video service increase, the necessity of the fourth generation wireless communication capable of improved broadband service is emerging in the situation where existing radio frequency resources are exhausted. Therefore, mm-wave communication using frequency resources in the 30 ~ 300 GHz band suitable for this is attracting attention as suitable for broadband communication system.

The wireless communication technology using the millimeter wave is the basic technology of the fourth generation wireless communication that enables broadband communication after International Mobile Telecommunication (IMT) -2000, but the existing broadband to overcome the problem that the transmission distance is limited or broadband is required. Interworking with wired network is necessary. Therefore, there is a need for element technology related to the connection of optical fibers and millimeter wave radios or the generation of optical millimeter waves.

Generating and transmitting optical millimeter waves in a mixed system of optical fibers and wireless devices has several advantages. As an example, when a desired millimeter wave is generated using light at a central base station and the millimeter wave light is converted into a wireless signal, a remote base station uses an antenna and an electrical-optic converter. By using this, the radio signal can be received. Therefore, the load of many antennas is reduced in a pico-cell communication network, and economical and efficient communication is possible.

In addition, a centralized system can be installed by imposing control functions such as channel assignments on the central base station. Since the baseband digital signal is transmitted without being modulated, the radio base station can be used for various types of radio waves. It is flexible in changing and establishing the radio type. In addition, the channel allocation and handover (hand-over) can be more easily controlled to improve subscriber capacity. Many functions necessary for antenna control for mobile communication are concentrated at the central base station, which can solve problems such as antenna positioning and maintenance.

Because of the advantages described above, the optical millimeter wave transmission method can be applied to a wireless subscriber network such as mobile communication or a broadband wireless local loop (B-WLL).

The method of oscillating millimeter wave signals using a light source in a fiber-wireless transmission system is classified according to the modulation scheme and the number of light sources.

Conventional millimeter wave signal oscillation methods include a modulation method, a direct modulation method (PAMorton, Electron. Lett., Vol. 30, no. 24, pp. 2044, 1994) or an external modulation method (U.Gliese, IEEE). Trans. Miceowave Th. Tech., Vol. 44, no. 10, pp. 1716, 1996). However, these methods have a problem in that due to the complexity and expensiveness of the electronic circuit, they are not economically feasible and oscillation is possible only in the low millimeter band.

As an oscillation method using one light source, a sideband technique method (GHSmith, IEEE Trans.Microwave Theory Tech., Vol. 45, no.8.pp. 1410, 1997), a mode locking laser (Mode Locking) Laser method (T. Kuri, IEEE Trans.Microwave Theory Tech., Vol. 47, no. 5, pp. 570, 1999), or Dual Mode Laser method (D. Wake, IEEE Trans.Microwave Theory Tech., Vol. 43, no. 9, pp. 2270, 1995). However, these methods are difficult to manufacture the optical device, and the lack of competitiveness in the development cost and price of the device is a problem of effectiveness.

Therefore, in consideration of price, frequency variability, and multi-channel scalability, an integrated form of two lasers is advantageous, and a heterodyne method of generating a signal of a desired frequency by beating between two light waves The oscillation method is valid.

Among the millimeter wave oscillation methods using heterodyne injection locking, L. Noel is a typical method (L. Noel, IEEE Trans.Microwave Theory Tech., Vol. 45, no. 8, pp. 1416 , 1997), which is a master laser that is a continuous wave in one of the sidebands of a radio-frequency-modulated slave laser. By injecting and locking, it causes the beating between the slave laser and the master laser to generate an optical signal of several tens of GHz. However, this method has a disadvantage in that the sideband power is so narrow that the choice of the sideband mode is narrow, so that the maximum achievable millimeter wave frequency is limited by the modulation performance of the master laser or the slave laser.

The technical problem to be achieved by the present invention is to inject a laser generated from the master laser into the slave laser to lock the four-wave mixing (Four-Wave-Mixing, FWM) conjugates generated by the slave laser, To make a device that oscillates waves.

In order to achieve the above technical problem, the optical oscillation apparatus of the present invention comprises an electrical signal generating device for generating a predetermined electrical signal; A laser driven by the electric signal to generate and output an optical signal of a first frequency (hereinafter referred to as a master laser); And a laser which is driven by the electric signal to generate an optical signal of a second frequency, receives an optical signal output from the master laser, and mixes the received optical signal with an optical signal of the second frequency (hereinafter, referred to as an optical signal). Slave laser).

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

1 is a block diagram of a frequency variable type optical oscillation apparatus according to the present invention. Reference numeral 10 denotes a light oscillator, and reference numeral 20 denotes a measurement unit 20 for measuring a spectroscopic characteristic of light output from the optical oscillator 10. The optical oscillator 10 includes an electric signal generator 11, a master laser ML, 12, and a slave laser SL, 13. An optical signal transmitter 15 may be further provided between the ML 12 and the SL 13, and a polarization controller 14 may be further provided between the ML 12 and the optical signal transmitter 15. .

The electric signal generator 11 supplies an electric signal capable of driving the ML 12 and the SL 13. The electrical signal is preferably an RF modulated signal. The ML 12 generates an optical signal modulated according to the electrical signal. The ML 12 is preferably an internally isolated distributed feedback laser diode (DFB-LD). The optical signal generated by the ML 12 is injected into the SL 13, and may be transmitted through an optical signal transmission device 15 such as an optical circulator. At this time, the optical signal generated by the ML 12 may be input to the light transmitting device 15 after passing through the polarization controller 14 for adjusting the polarization state of the optical signal to reduce the power loss. The SL 13 is suitably DFB-LD or Fabric-Parot LD (FD-LD) without an internal isolator. At this time, in order to induce a non-degenerate FWM phenomenon in the SL 13, the frequency f _ML of the optical signal generated in the ML 12 is located outside the locking area of the SL 13 and the SL ( It is to be detuned so that down-conversion occurs with the SL 13 by being located in a higher frequency region than the frequency f _SL of the optical signal generated in FIG. 13. When the continuous wave is generated in the ML 12 by adjusting the operating temperature or bias current of the ML 12 and injected into the SL 13, the non-linearity characteristic of the semiconductor laser causes f _ML and f _SL. Non-degenerate FWM conjugate signals with different frequencies in accordance with the combination of. At this time, the photon density in the cavity of the SL 13 oscillates at a frequency that is about the difference between the lasing frequency of the ML 12 and the SL 13. It is possible to detect beat signals between these signals using a direct detector that complies with the law.

The measuring unit 20 includes an optical isolator 21, an optical amplifier 22, an optical coupler 23, a photo detector 24, an RF measuring device 25, and an optical signal measuring device 26.

The optical isolator 21 prevents the optical signal transmitted from the light transmitting device 15 from flowing back toward the light transmitting device 15. The optical amplifier 22 amplifies the optical signal passing through the optical isolator 21, and the optical coupler 23 divides the amplified optical signal into two optical signals. The photo detector 24 converts one of the divided optical signals into an electrical signal. At this time, the detecting bandwidth of the photodetector 24 should be larger than the millimeter wave frequency to be generated. The RF signal measuring device 25 measures the output signal of the photo detector 24 in the RF region. An RF-spectrum analyzer is suitable for the wireless signal meter 25.

The other signal divided by the optical coupler 23 can be optically measured using the optical signal meter 26. The optical signal measuring device 26 is preferably an optical spectrum analyzer or a Fabri-Perot Interferometer having a low free spectrum range.

In this case, the electrical signal generator 11 may generate frequency-modulated signals at the ML 12 and the SL 3 to generate signals of the side bands around each of the rising frequencies. The signal can be further stabilized. This will be described in more detail as follows. In the direct detector according to the square law described above, in addition to the ML 12, SL 13, or FWM signals of the oscillator 10, the bit signals generated by the beats of these signals are detected. Coherency is weak because it is generated by other driving sources, resulting in poor stability or purity. In order to stabilize the bit signal and reduce phase noise, the ML 12 and the SL 13 are connected to a single electrical signal generator 11, preferably an RF signal source through a 3 dB coupler (not shown). (RF-source), the RF signal source can supply an electrically direct-modulated signal to the ML (12) and SL (13). In the ML 12, sideband modes are formed around the lasing frequency by frequency modulation due to an RF signal source, and these modes are injected into the cavity of the SL 13. Combined with the modes, you get a locking effect. As a result, not only the fundamental mode of the SL 13 but also the non-degenerate FWM conjugate modes are shifted and locked in superimposition to the sideband mode of the ML 12, resulting in a signal with less fluctuation and phase noise. Can be obtained.

FIG. 2 (a) shows the continuous wave output from the ML 12 and the SL 13, respectively, and the peak of the FWM conjugate which appears when the two continuous waves are interacted with each other. The interval fb between the FWM conjugate frequencies fI, fJ and f _ML , f _SL is equal to the interval between the output frequencies of the ML 12 and SL 13. The frequency f _SL of the optical signal output from the SL 13 is lower due to a decrease in the carrier density of the SL 13 when the light output from the ML 12 is injected into the cavity of the SL 13. This results in a red-shift phenomenon that shifts to frequency. At this time, since the light injected into the SL 13 acts as a pump laser causing the FWM rather than the master laser for locking, the bit signals generated from the ML 12 and the SL 13 may be fluctuated. Phase noise is a very unstable signal. In the laboratory, beat signal frequency fluctuations, typically on the order of tens of MHz, are measured.

2 (b) and 2 (c) show the spectra of light output from the ML 12 and the SL 13 when they are electrically RF modulated. In the case of direct modulation, not only intensity modulation but also frequency modulation is caused, so that sideband modes occur at intervals of the RF modulation frequency fm. FIG. 2 (d) shows the red shift of f _SL in SL 13 due to injection locking. That is, the sideband modes of the FWM of the ML 12 injected into the SL 13 overlap with some of the sidebands of the SL 13 to cause an injection locking effect. As a result, the sideband modes of f _ML are injected and as they turn red the position where f _SL is not a multiple of the RF modulation frequency from f _ML is a multiple of the RF modulation frequency around f _ML (f _SL Move to ') and lock. The red shift phenomenon becomes more severe as the RF modulated signal power Pm increases, and the locking strength due to sideband mode and coupling is also improved.

As f _SL moves to f _SL ', the corresponding FWM conjugates fI and fJ not only move to fI' and fJ ', but also are locked to each other. As f _SL moves, the spacing between FWM conjugates is also adjusted to a multiple of the RF modulation frequency fb 'at fb, so that the entire FWM conjugates are locked to each other to generate a bit signal with low phase noise and stable frequency.

3 (a) to 3 (f) show the results of measuring the output of the SL 13 with a light spectrum analyzer when the light output from the ML 12 is injected into the SL 13 according to an embodiment of the present invention. It is shown. 4 (a) to 4 (f) show the results of measuring the output of the SL 13 with an RF spectrum analyzer when the light output from the ML 12 is injected into the SL 13 according to an embodiment of the present invention. It is shown. 3 (a) and 4 (a) show the case where RF modulation is not performed, and FIGS. 3 (b) to 3 (f) and 4 (b) to 4 (f) show RF modulation signal power Pm of 5 dBm, respectively. The results for 8 dBm, 10 dBm, 12 dBm and 16 dBm are shown.

3 (a) and 4 (a), in the absence of RF modulation, the output spectrum was detected with f _SL , f _ML , FWM conjugate, and a bit signal around 13.8 kHz.

It can be seen that when the modulation frequency fm increases the power Pm of the RF modulated signal of 3 GHz, f _SL moves to a lower frequency than before, as shown in FIGS. 3 (b) and 3 (c). The red shift of the lasing frequency of the SL 13 according to the increase of Pm may be seen as the increase in the bit signal frequency in FIGS. 4 (b) and 4 (c).

Due to electrical modulation, other bit signals may be measured as far as fm from the bit signal in the 15 GHz frequency band in FIG. 4, which corresponds to higher order harmonics of fm. These beat modes shown in FIGS. 4 (b), 4 (c) and 4 (f) are shown without overlapping with the beat signal mode of 15 GHz, in this case the bit signal of 15 GHz. It can be seen that is a higher-order harmonic that simply corresponds to an integer multiple of fm without locking.

The peripheral modes shown around the bit signal in Figures 4 (c) and 4 (f) can be considered to be due to back-reflection occurring at the fiber pigtail surface in the SL without internal isolators.

Increasing Pm, the sideband modes of the SL become redshifted and locked by the sideband mode of ML, as shown in FIGS. 3 (d) and 3 (e), so f _SL is 15 GHz, which is a multiple of fm from f _ML . It is located about a distance away.

4 (d) and 4 (e) show that a locked bit signal is generated that is stable and has low phase noise without other peripheral modes around the desired bit signal. For the signal shown in Fig. 4 (e), the phase noise was measured at -96 dBc / Hz on the spectral position with a 100 kHz offset from 15 GHz. In addition, the line width of the unlocked bit signal of FIG. 4 (a) is about 4 MHz, but the line width of the locked bit signal is limited only by the resolution of the RF spectrum analyzer. Therefore, it can be seen that the optical oscillator according to the present invention also contributes to reducing the line width of the signal. As shown in Figs. 4 (d) and 4 (e), the power of the bit signal has been markedly increased, indicating that the bit signal is not simply a higher-order harmonic of fm but a result of locking. As long as the sideband modes of the SL are locked, it can be seen that f _SL is not biased even if Pm is increased. In the region where Pm is 10dBm to 13.5dBm, the SL maintains the locking behavior. . If Pm exceeds 13.5 dBm, as shown in Figs. 3 (f) and 4 (f), the SL is not locked and again becomes redshifted further.

5 illustrates the relationship between the bit signal frequency and the power of the modulated signal. The square points in FIG. 5 represent the measured FWM bit frequency, and the solid line represents the result of linear fitting to the FWM bit frequency without considering locking. As shown, as long as the sideband modes of the SL are locked in the locking region, it can be seen that a stable bit signal of 15 GHz is generated without frequency fluctuation. Also, since the SL sideband signals are not locked, it can be seen that the bit signal frequency monotonically increases with increasing Pm, and the frequency changes with a slope of 150 MHz / dB.

Therefore, in the Pm locking region of about 10 dBm to 13.5 dBm, a millimeter wave of high frequency with low phase noise and stable frequency can be generated. If the bit signals between the FWM conjugates are used, signals of 30 GHz, 45 GHz, and 60 GHz frequencies may be obtained, and the frequency band of the generated signal using the present invention is very wide.

The optical oscillator according to the present invention may vary the frequency of the generated signal. By adjusting the lasing frequency by controlling the bias current or operating temperature of ML, the frequency of the generated signal due to beating between FWM conjugates can be adjusted. This method has coarse tuning due to the wide tunable range. Suitable for

In addition, it is also possible to vary the frequency generated by adjusting the RF modulation frequency, which is the sideband mode of the ML generated by the frequency-modulated RF modulation signal, the fundamental frequency mode of the SL and its sideband mode is also FWM conjugate The frequency of occurrence can be varied by locking the sideband modes of.

 This method is easy to lock, making it suitable for fine tuning with resolutions of tens of MHz. When the above two generation frequency variable methods are used together, the generation frequency can be adjusted within several GHz bands.

According to the present invention, it is possible to generate an optical signal having a frequency in a wide range from tens of MHz to several THz as an ultrafast optical signal generation technique through high speed modulation. In addition, based on this, it is possible to implement an optical device such as a semiconductor laser or an optical modulator required in a high speed optical communication system. The optical oscillation device according to the present invention can vary the frequency, and can be controlled at the same time in several GHz and finely adjusted at intervals of several tens of MHz at the same time by controlling the current, temperature, etc. of the laser diode or adjusting the frequency of the electric modulation signal. Implementing light sources improves the frequency variability of broadband wireless communication systems, such as those installed in broadband-wireless local loops (B-WLLs) and has a wide range of industrial applications. Stabilization of the light source can be achieved by reducing phase noise of the light source and reducing fluctuations in the generated frequency. It also reduces 3rd order intermodulation distortion and improves spun-free dynamic range (SFDR) for high-quality communication, and reduces transmission dispersion due to reduced chirp of the light source, which reduces performance during transmission. little. In particular, this method uses locking between FWM conjugate modes, so that the range of available harmonic modes is wide, enabling a stable ultra-high frequency light source without an upper limit on the frequency of the generated signal. If the light source according to the present invention is adopted as the millimeter wave light source of the central station, the load of the antenna of the remote base station can be reduced, thereby miniaturizing the base station or the mobile terminal, and necessary functions for antenna control for mobile communication. It is possible to construct a system in which the centralized base station and the facilities are centralized, so that antenna maintenance, channel allocation control and handover control are easy.

In addition, it is easy to prepare for the versatility of the propagation format, and thus it is possible to have flexibility in changing and establishing the propagation format.

Claims (7)

An electrical signal generator for generating a predetermined electrical signal; The laser is driven by the electrical signal to generate an optical signal of the first frequency and the optical signal of the first sideband modes spaced by the modulation frequency interval of the electrical signal around the first frequency (hereinafter referred to as a master laser) Called); And The optical signal is driven by the electrical signal to generate an optical signal of a second frequency, and generates an optical signal of second sideband modes spaced apart by a modulation frequency interval of the electrical signal about the second frequency, and outputs from the master laser. And a laser (hereinafter referred to as slave laser) for receiving the received optical signal and mixing the received optical signal with the optical signal of the second frequency. The apparatus of claim 1, wherein the electrical signal generator An optical oscillator, characterized in that the radio frequency modulator for generating an electrical signal modulated with a radio frequency (RF). The method of claim 1, wherein the master laser An optical oscillation device characterized in that it is an internally isolated distributed feedback laser diode. The method of claim 1, wherein the slave laser is An optical oscillation device characterized by a distributed feedback laser diode or a Fabry-Perot laser diode that is not internally isolated. The method of claim 1, wherein the master laser And an optical signal having a frequency belonging to a high frequency region rather than a frequency of the optical signal generated by the slave laser. The method of claim 1, And an optical transmission device positioned between the master laser and the slave laser to transmit the optical signal output from the master laser to the slave laser, and to transmit the optical signal output from the slave laser to the outside. Optical oscillation device. The method of claim 6, And a polarization controller positioned between the master laser and the optical signal transmission device to adjust a polarization state of the optical signal output from the master laser.

Images