GB2478746A

GB2478746A - Radio over fibre system with direct modulation of a DC biased laser by a data modulated RF signal, preferably producing an optical comb signal

Info

Publication number: GB2478746A
Application number: GB201004354A
Authority: GB
Inventors: Liam Barry; Prince Anandarajah; Philip Perry; Haymen Shams
Original assignee: Dublin City University
Current assignee: Dublin City University
Priority date: 2010-03-16
Filing date: 2010-03-16
Publication date: 2011-09-21
Also published as: GB201004354D0

Abstract

The present application provides a system and method for providing RF data over a fibre using direct modulation of a laser. A data signal 42, RF signal 22 and direct current bias (26, Fig. 2) are combined to produce a drive current to the laser 45. Preferably, the laser is configured to generate a comb output having a plurality of sidebands amplitude modulated with the data. The sidebands are filtered 46 to preferentially select two sidebands having a desired RF frequency separation. The filtered signals are then transmitted by fiber to a receiver where the two sidebands beat together at the RF frequency providing an amplitude modulated RF signal for onwards transmission. In order to generate the optical comb signal the laser may be driven with a DC current close to the threshold current or/ and the RF signal may have a frequency close to the relaxation frequency of the laser.

Description

A DIRECT OPTICAL MODULATOR

Field

The present application relates generally to the optical distribution of data signals.

Background

Radio systems have been developed that can provide wireless communication with bit rates in excess of 1Gbps, but are typically constrained to relatively short range paths of the order of metres. Radio-over-fibre (ROE) techniques exist to modulate radio signals onto optical carriers so that a central station may distribute the radio signals to a number of remote antenna units. Such systems typically can not work at radio frequencies higher than 20GHz as they are limited by the bandwidth of the lasers, although the use of external modulators can extend this to approx 40GHz. However external modulators add cost.

Additionally, they are limited to approx 40GHz.Techniques that can provide this radio over fibre functionality at millimetre wave frequencies include frequency multiplication, frequency up-conversion.

However, the present application is directed at the use of an optical comb source.

One generally employed technique of generating an optical wavelength comb is to employ an amplitude modulator pair. An example of such a technique is provided in Fatima C. Garcia Gunning and Andrew D. Ellis "Generation of a widely spaced optical frequency comb using an amplitude modulator pair" Opto-Ireland 2005: Optoelectronics, Photonic Devices, and Optical Networks, Proc. of SPIE Vol. 5825 (SPIE, Bellingham, WA, 2005). An example of such a wavelength comb generator device 1, as shown in Figure 1, comprises a laser source 2, the output of which is fed through two modulators 4 and 6 in series to provide a wavelength comb output 8. To obtain this output, the modulators are in turn driven by a sinusoidal signal provided from an oscillator 16, for example approximately 40GHz, which is amplified by amplifiers 10, 12 providing drive signals to the modulators. The sinusoidal signal provided to the second modulator may be phase shifted by a phase shifter 14.

Whilst these techniques are effective, a problem with these existing comb generation techniques is that the modulators are expensive and bulky.

"Optical millimeter-wave generation and transmission system for 1.25 Gbit/s downstream link using a gain switched laser" by H. Shams et al. [Optics Communications 282 (2009) 4789-47921 describes an alternative approach at a system overview level in which a gain switched laser (GSL) is employed to produce an optical comb spectrum that can be appropriately filtered to generate two optical sidebands.

Using On-Off keyed modulation, data is imposed upon the optical sidebands and the resulting modulated signal is transmitted by fiber. Whilst, the system is broadly described, there is insufficient detail to implement a working system in the paper. In particular, there is no disclosure of how the laser is operated to produce the required comb. The system uses an external amplitude modulator to apply the data to the optical signal, which consumes power, occupies space and adds cost in a system.

The present application is directed at overcoming one or more of the aforementioned problems.

Summary

Accordingly, the present application provides an optical transmission device, a radio over fiber communications system and a method in accordance with the claims which follow.

Description of Drawings

Figure 1 is an example of a prior art approach to using a generated comb to transmit data, Figure 2 is an exemplary comb generator device according to the present application, Figure 3 is an exemplary test set-up for use with the comb generator device per the arrangement of Figure 2, Figure 4a and Figure 4b are exemplary results obtained using the set-up of Figure 3, Figure 5 illustrates the mode of operation of a conventionally operated pulse laser with (a) applied current (b) carrier density and (c) output optical pulses shown separately, Figure 6 is an exemplary use employing the arrangement of Figure 2, Figure 7 is a system level view of a system employing the arrangement of Figure 6, Figure 8 is a less detailed view of Figure 6 in which the optical output at different points in the system is shown, Figure 9 illustrates the optical output produced by the laser of Figure 6 before and after filtering, and Figure 10 illustrates some experimental results from an experimental set-up corresponding to Figure 6.

Detailed Description

The present application employs a semiconductor single mode laser, including for example but not limited to ridge lasers.

In common with prior art techniques, the laser may be maintained at a preset temperature using appropriate circuitry and devices, including for example a Peltier device. As is known in the art, the frequency of the laser may be altered by adjusting its operating temperature.

In contrast to the techniques of the prior art which sought to produce a comb output by using one or more external optical modulators and then modulate the resultant comb with an RF signal that was already modulated with the data signal to be transmitted by using another external modulator. The present application imposes the data directly onto a gain switched laser so that the comb produced by the laser is already modulated. Such an approach is made possible by careful selection of the frequency and power of the modulation signal and the level of the DC bias. The resulting signal applied to the laser yields pulses in the temporal domain with a comb of frequencies in the spectral domain, i.e. the comb is produced by altering the electrical signals provided to the laser rather than by modifying the light output from the laser as would be the conventional technique employed in the art. Moreover, as the comb produced is already modulated with the data signal, there is no need for a modulator to modulate the comb produced. It will be appreciated that the complexity, size and cost of the biasing and modulation circuits are considerably less than the equivalent hardware required for the prior art post lasing techniques.

More particularly, the essence of the present application, which is shared with a co-pending but unpublished application (GB0904831.5 filed 20th March 2009) of the present assignee, is that the laser is operated at a frequency close to its relaxation oscillation frequency, so that the build up of each pulse occurs close to the end of the preceding pulse, so that coherence is maintained between consecutive pulses. This effect causes the inter-pulse timing jitter to be minimised and the phase relationship of the optical signal to be maintained between the consecutive pulses. The frequency domain of this periodic pulse train therefore yields a comb of low line width tones that are phase coherent. Moreover, this technique allows for modulation to be imposed prior to rather than post generation of the optical signal.

The mode of operation may be further explained by contrasting the mode of operation with that of a conventionally operated gain switched laser and figure 5 in which is illustrated the typical evolution of the photon and electron density, which takes place during the generation of ultra-short optical pulses by gain-switching a laser diode.

The laser is biased with a DC current (Is), which is below threshold (Ith) as shown in 5(a). A large amplitude pulsed current is then applied to the laser, which is also shown in the same 5(a). Since the laser is biased below threshold, the initial photon density is very low and since the stimulated emission rate is proportional to the photon density, the photon density increases at a very slow rate. In the absence of a sizable amount of stimulated emission, the carrier concentration increases rapidly in the laser. When the electric pulse increases the injected carrier density above the carrier density threshold (flth), lasing starts. The typical time development of the carrier density 5(b). At a certain point (peak inversion point [n1]) the generated photon population rapidly depletes the electron concentration. If the current pulse is cut off at the appropriate time, as the photon and electron densities are decreasing after the initial peak, then the second oscillation will not be obtained. That is to say that above flth, the carrier density reaches n1, lasing occurs and represses the increase in carrier density and consequently n1 is pulled down to n1. Therefore further lasing is prevented if the current is abruptly terminated after the charge carrier concentration is exceeded so that only a single resonance spike is generated (preventing further relaxation oscillations to occur). If the current pulse is not cut off the photon density will then oscillate at the resonant frequency before settling down to its steady state value. The present application in common which the previously referenced GB0904831.5 seeks to drive the laser with a signal having a frequency close to that of the relaxation oscillations An exemplary circuit arrangement 20, as employed in GB0904831.5, for driving a laser 30 to provide an output having a comb frequency characteristic, as shown in Figure 2, comprises a bias circuit 26 which is used to provide a DC bias. DC bias circuits would be familiar to those skilled in the art. The level of the DC bias is selected to provide a current close to the threshold current of the laser. Suitably, the DC bias is selected to be just below the threshold of the laser. It will be appreciated that the level of bias required will be dependent on the laser, but a typical value for the bias would be of the order of l0mA to 60mA. An oscillator 22 provides a RE signal which is amplified by an RE amplifier 24 and combined with the DC bias in a combining circuit 28, which may for example be a bias tee.

In order to determine suitable operating parameters and\or to optimise the output of the laser a test set-up may be employed for example as shown using the set-up of Eigure 3, by connecting the output of the laser to a Optical Spectrum Analyser with suitably fine resolution (in the MHz region) and adjusting the dc bias (not shown) and the RE drive level and frequency to maximise the depth between the lines of the comb. This improves the contrast between the lines of the comb and the in-band noise. This inherently maximises the phase coherence between the lines and frequency stability of each individual line. It will be appreciated that this optimisation may be performed once when characterising a particular model of laser and then the values preset for the purposes of general manufacture.

Eor the purposes of illustration, some exemplary results obtained using the set-up of Eigure 3 are shown in Eigures 4a and 4b. As can be seen from the generated pulse train of the laser shown in Eigure 4a and its corresponding optical spectrum shown in Eigure 4b a comb spectrum was generally produced. To verify the performance of the exemplary circuit, the linewidth of the individual tones were measured by removing the other tones with a Eabry-Perot optical bandpass filter. In one particular case, the linewidth of individual lines was measured to be 3.8MHz. The Iinewidth of the complete signal was also measured with all the tones, yielding a linewidth of 4.5MHz. To obtain such a result for the entire comb, each of the individual tones MUST be substantially coherent with all the others. In the case of an incoherent comb, the resultant linewidth is calculated as the linewidth of the individual lines, multiplied by SORT (number of lines). So the 13 tones in this case which would give a total linewidth of 13.7MHz. It will be appreciated that such a technique may be employed to test generally as to whether a laser has been successfully configured to generate a wavelength comb.

Whilst this technique is effective at producing an optical comb, it still requires the use of a subsequent modulator to modulate the comb with data.

The present application obviates the requirement for an external modulator. In particular, the present application may be explained with reference to the previously described technique whereby if the bias signal is switched on and off, the laser will produce a coherent comb when the bias is on and not when it is switched off. The present application uses this characteristic to modulate the data directly. However rather than modulate the bias current, the present application combines the data signal with the RE signal and then applies the combined signal with the bias to drive the laser. Thus when the data is a particular level (high) a comb is produced whereas when the opposite level (low) is present a comb is not produced. Thus it will be appreciated that the comb produced by the laser is modulated by the data signal.

The present application will now be described in greater detail with respect to an exemplary arrangement 80 in which data is transmitted from a central station 40 to one or more remote antenna units 57 where the data signal is transmitted via a wireless link to one or more receivers 62. The exemplary arrangement, as shown in some detail in Figure 6 and in Figure 7 at an overview level, comprises an RE signal generator 22, which as described previously may, for example, be a voltage controlled oscillator, possibly stabilised with a Phase Locked Loop, providing a RE output signal. The RE signal is selected to correspond approximately to the relaxation oscillation frequency of the laser. The RE signal is suitably sinusoidal. The relaxation oscillation frequency of the laser may be determined roughly by finding the peak in its frequency response which occurs at a frequency slightly less than the RE bandwidth of the laser. The RE signal is amplified by a first RE amplifier 24 to provide an amplified RE signal as an output.

A data source 42 provides the data signal. The data source may internally generate the data signal or accept it from another source, for example media gateway 82. The data source suitably phase locks the data signal to that of the RE signal. The data may be in any binary form. The data signal is amplified by a second RE amplifier 44 to provide an amplified data signal as an output.

The outputs of the first and second amplifiers are combined together in a combiner 45. A bias current (not shown) provided from a bias circuit (not shown) is also combined with these signals. It will be appreciated that a separate combiner may be employed to combine the data and RE signals first and then a second combiner (e.g. the bias T previously described or similar circuit structure) may be employed to combine the bias current with the previously combined data and RE signals. The output from the combiner is used to drive the laser. As shown in Figures 8 and 9, the output from the laser 30 is a comb with a plurality of sidebands, where the sideband separation represents the frequency of the RE signal. As represented, each sideband is modulated by the data.

For the exemplary embodiment for transmission of radio data at a particular frequency, the optical output (comb) from the laser is filtered by one or more filters 46 to suppress most of the sidebands of the comb. The filters are designed to preferably select two sidebands from the comb. The two side bands are selected by a) having a desired frequency separation between them and b) being convenient for filtering such that the remaining sidebands are suitably suppressed with respect to them after filtering. Thus an optical band pass filter (50) may be employed. The optical band pass filter may be selected to suppress sidebands below the lower of the two sidebands. Similarly, the optical band pass filter may be selected to suppress sidebands above the higher of the two sidebands. An optical band stop filter (48) may be employed to suppress side bands between the two preferably selected sidebands. Thus in the example of Figure 8, the 3rd and 7th sidebands are preferentially selected with the 1st 2 8th and 9th sidebands being suppressed by the band pass filter with the 4th, 5th and 6th sidebands being suppressed by the band stop filter (In Figure 8 the two filters are shown as one for ease of representation). The filters may for example be Bragg filters. The preferential selection of the two side bands is not essential for the transmission of the data but is advantageously employed in the circuitry of the remote antenna unit (described below).

An optical amplifier 52 may be employed to amplify the optical signal coming from the filters. A suitable amplifier would be an erbium doped fiber amplifier (EDFA). The resulting amplified signal may be transmitted over a waveguide 54, for example an optical fiber such as a standard single mode fiber (SSMF). One or more optical splitters 55 may be used to provide the optical signal to a plurality of fibers which in turn are provided to a plurality of optical receivers 57 (in the exemplary system remote antenna units).

At the receiver (remote antenna unit (RAU)), the optical signal is received. A photodetector 56 is employed as a detector of the optical signal. The two sidebands beat together in the receiver and generate a modulated signal having a central frequency corresponding to the difference in frequency between the side bands. Thus, in the example above where the 3rd and 7th sidebands were preferentially selected if the RE signal was selected, a 60GHz signal would be produced. It will be appreciated that the 60GHz signal will have the data signal modulated upon it. The modulated signal may then be amplified by a suitable amplifier 58 (e.g. a broadband amplifier) in the receiver. The resulting amplified signal is provided to an antenna 60 where it is transmitted as a radio signal.

This radio signal in turn may be received by an antenna 64 of a wireless receiver. Upon reception at the wireless receiver, conventional techniques may be employed to extract the data signal. For example, the received radio signal may be amplified by a RF amplifier 72 and then mixed in a mixer 70 with a frequency generated by a local oscillator 66. As would be understood by those skilled in the art, the oscillator frequency is selected to correspond to that of the RF signal. A Low Pass Filter 68 (having a cut-off frequency above that of the bandwidth of the data signal) is then employed to extract the data signal which may be amplified by a further amplifier 74. The data signal may then be processed using conventional techniques.

The above described exemplary system employs a technique based on a gain switched laser (GSL) for optical generation and transmission of mm-wave that does not require any external modulator. The elimination of external modulators which is enabled by this architecture facilitates creating low cost systems that can bring optical distribution of 60 GHz signals to a commercial reality. As described gain switching is achieved by driving the laser with a large RF signal. In the system described above, the RF signal is produced by coupling the sinusoid signal with data (suitably non-return to zero (NRZ) data). The GSL generates multiple phase correlated sidebands, spaced by the driving RF frequency and modulated with data. By using the previously described filters to select specific components of the comb, the generated mm-wave signal can be at many times the drive RF signal frequency. In the exemplary system, the selected sidebands are transmitted over an optical network containing passive optical splitters to a number of remote antenna units (RAU5). These components beat together at the detector in the RAU to yield an amplitude modulated mm-wave signal. This signal is then amplified and transmitted to the mobile units (MU5) using an antenna. Using this method, the present inventors have successfully implemented the transmission of 1.25 Gbps downstream data over 3 km fiber with 2 m wireless distance on a 60 GHz RF carrier. The system performance has been measured by using a bit error rate (BER) detector, and the eye diagrams have been recorded after transmission. The results have shown that this system can easily generate mm-waves with a stable spectrum with less cost and complexity than comparable systems.

In the experimental system, a commercial distributed feedback laser diode (DFB-LD) with an emission wavelength of 1551 nm at room temperature with a threshold current of 15 mA. The DFB-LD was biased at 43 mA and gain switched by an RE signal. The RE signal was composed of an 15 GHz sinusoidal signal combined with 1.25 Gbps downstream data (pseudo-random bit sequence with a 27-1 word length) generated by using a pulse pattern generator synchronized with the 10 MHz reference from the signal generator (SG). The generated RE signals are shown as inset (i) in Eig. 6, and it can be seen that each temporal bit slot consists of twelve cycles of the 15 GHz sine wave. The DEB-LD was externally injected with another DEB-LD to decrease chirping and the time jitter of the gain switched pulses. The generated gain switched spectrum is captured by a high resolution optical spectrum analyzer (OSA) and is shown in Eig. 9(a). The spectrum shows the generated comb with 15 GHz spacing between comb lines, and each tone modulated by the 1.25 Gbps data stream. The optical spectrum consists of nine central sidebands covering a spectral range over 120 GHz, with a maximum power difference between the comb lines of about 5 dB. This optical spectrum is filtered by using an optical band stop filter (OBSE) to suppress three tones in the middle and an optical band pass filter (OBPE) is used to reject the outer sidebands. The resultant output spectrum is illustrated in Eig. 9(b) and it shows the two main optical tones spaced by 60 GHz which are clearly modulated with the data stream. The suppressed sidebands are shown around 15 dB below than the main sidebands. In the experimental set-up the optical filters available at the time were non-ideal and much better suppression may be achieved by employing a specially designed Bragg filter to select the two required sidebands as previously described. The filtered optical signal as captured by an oscilloscope is shown as an inset (ii) in Eig. 6. The extinction ratio between 1 and 0 bits was adjusted to give the best performance. More specifically, a tuning process was conducted in which the DC bias to the laser was varied along with the peak to peak voltage of the data signal and the peak to peak voltage of the RE sine wave to a point where the results were satisfactory. More particularly, it was found that if the peak to peak voltages were too large then the 0 level data is completely switched off which resulted in noisy pulses (poor comb).

This is beneficial because as the extinction ratio is increased the level of timing jitter and noise on the optical pulses increases, which reduces overall system performance.

As described previously, after filtering, the optical signal was amplified by using an erbium doped fiber amplifier (EDEA) and then transmitted over standard single mode fiber (SSME) to the RAU. At the RAU, the optical signal was photodetected by a high speed photodiode with a 3 dB bandwidth of 50 GHz. The two sidebands beat together in the receiver and generated a modulated 60 GHz mm-wave. The converted electrical signal was subsequently boosted by a mm-wave amplifier to compensate for the limited bandwidth of the detector. Afterwards, the mm-wave signals were broadcast to an MU via a 20 dBi horn antenna.

At the MU, the mm-wave signal was received by an identical horn antenna, amplified, and mixed with a GHz LO to down convert to a base band signal. The signal was then filtered by using a low pass filter (LPF) and amplified again. The demodulated 1.25 Gbps signal was detected by a bit error rate tester (BERT) and the eye diagrams were monitored by using a high digital speed sampling oscilloscope (OSC).

In Figure 10, the measured BER is plotted versus the received optical power for back-to-back (B2B), and 3 km fiber transmission with and without wireless transmission. The eye diagrams of the recovered baseband signals are also shown as insets in Fig. 10. The inset (i) in Fig. 10 exhibits the eye diagram for B2B without wireless transmission at received power of -30.9 dBm. As can be seen from the figure, the B2B receiver sensitivity for BER of 10-9 is -31.9 dBm and there is 0.7 dB power penalty after 3 km fiber transmission without wireless transmission. For B2B optical connection and 2 m wireless transmission, the receiver sensitivity was degraded by 4.4 dB to -26.5 dBm due to the signal to noise ratio degradation in the radio system. For a combined 3 km fiber and 2 m wireless scenario, the receiver sensitivity is further degraded to about -25.2 dBm, and the eye-diagram in this case is shown as inset (ii) in Fig. 10 for a received power -24.4 dBm.

Accordingly it has been demonstrated that the system described herein is effective and may be employed to reduce the cost of providing a system for the commercial deployment of short range distribution of data, for example, in home or business buildings.

Claims

Claims 1. An optical transmission device for transmitting a data signal using a laser, the transmission device comprising: a biasing circuit for providing a DC bias, an RE circuit providing a RE signal, a drive circuit for combining the data signal, RE signal and bias to provide a drive current to the laser.
2. An optical transmission device according to claim 1, wherein the laser bias current and DC offset in the data signal are selected to ensure that the laser is driven with a DC current close to the threshold current of the laser device.
3. An optical transmission device according to claim 2, wherein the laser bias current and DC offset are selected to ensure that the laser is driven with a DC current within 20% of the threshold current.

Q
4. An optical transmission device according to any preceding claim, wherein the RE signal comprises a substantially sinusoidal signal. C)
5. An optical transmission device according to any preceding claim, wherein the frequency of the RE signal is close to the relaxation oscillation frequency of the laser.
6. An optical transmission device according to claim 5, wherein the frequency of the RE signal is within 25% of the relaxation oscillation frequency of the laser.
7. An optical transmission device according to any preceding claim, wherein the frequency of the data signal is less than 20% of the RF signal.
8. An optical transmission device according to any preceding claim further comprising an optical splitter to provide a plurality of outputs from the optical transmission device.
9. An optical transmission device according to any preceding claim where the laser produces a comb output having a plurality of sidebands.
10. An optical transmission device according to claim 8, further comprising at least one optical filter for preferentially selecting one or more of the sidebands.
11. An optical transmission device according to claim 10, wherein the at least one optical filter is configured to preferentially selects two sidebands.
12. A radio over fiber communications system comprising an optical transmission device according to any preceding claim and at least one fiber wherein the optical output from the optical transmission is transmitted through said fiber to a local antenna unit.
13. A method of operating a laser to provide an optical signal carrying a data signal, the method Qis comprising the step of driving the laser with a DC bias current, a data signal and a RE signal.C')
14. A method according to claim 13, wherein the RE signal comprises a substantially sinusoidal signal.co
15. A method according to claim 13 or 14, wherein the frequency of the RE signal is selected to be close to the relaxation oscillation frequency of the laser.
16. A method according to claim 15, wherein the frequency of the RE signal is within 25% of the relaxation oscillation frequency of the laser.2S
17. A method according to anyone of claims 12 to 16, wherein the bias current is selected to be close to the threshold current of the laser device.
18. A method according to claim 17, wherein the bias current is within 20% of the threshold current.
19. A method according to any one of claims 12 to 18 wherein the output of the laser is coherent.
20. A method according to any one of claims 12 to 19, wherein the laser is temperature stabilised.
21. A method of manufacturing an optical transmitter for transmitting a data signal comprising the initial steps of selecting a type of laser, biasing one of the type of laser with a DC bias current and an RE signal, adjusting the frequency of the RE signal to determine a frequency of the RE signal to produce a comb output, and further comprising the further steps of fabricating a laser device, the fabricated laser device comprising: a bias circuit having an output to provide a DC bias current, a RE signal generator having an output to provide a RE signal, a modulator for modulating the RF signal by the data signal to provide a modulated RE signal, and a drive circuit for combining DC bias current and the modulated RE signal to provide a drive signal to the laser, wherein the RE signal generator is configured to operate close to the determined frequency.
22. A method according to claim 21, further comprising as an initial step the adjusting of the bias C') current to determine an optimum bias current for the desired comb output and wherein the DC bias circuit is configured to operate at the determined optimum bias current. C)23. A method according to claim 21 0122, wherein the initial frequency of the RE signal is selected to be approximately the relaxation oscillation frequency of the laser type.