OPTICAL MODULATOR

Description

Optical modulator The invention relates to an optical modulator and to a method for operating an optical modulator. In addition, a communication system comprising at least one such modulator is suggested . The fast growth of data exchange has led to an increasing de^¬ mand for transmission bandwidth. Dense wavelength division multiplexing (DWDM) networks offer high capacity transmissi^¬ on, both in core and/or backhaul and access and/or metro net^¬ works .

Cost efficient DWDM networks are enabled by the use of advan^¬ ced modulation formats, such as differential binary phase shift keying (DPSK) , differential quadrature phase shift key^¬ ing (DQPSK) , duobinary, carrier-suppressed return-to-zero (CS-RZ) , or optical single sideband (OSSB) , etc.

To generate optical signals with such modulation formats, it is required to control the phase and amplitude of the optical signal. For such purpose Mach-Zehnder modulators (MZMs) are commonly employed; however, the MZMs have high insertion losses (around 6 dB) and are considered expensive for access and radio-over-fiber applications. Directly modulated lasers have been utilized for such purposes, but the resulting amplitude modulated signals also present undesired phase distortion or reduced amplitude extinction ratio.

The problem to be solved is to avoid the disadvantages stated above and in particular to provide a cost-efficient solution of a modulator that is capable of modulating the amplitude or the amplitude and phase. The modulator may further be based on SOA technology. This problem is solved according to the features of the inde^¬ pendent claims. Further embodiments result from the depending claims .

In order to overcome this problem, an optical modulator is provided

- comprising at least two semiconductor optical amplifier sections that are arranged in a cascaded struc^¬ ture ;

- wherein an information signal is applicable to one of the semiconductor optical amplifier sections and an inverse information signal is applicable to another of the semiconductor optical amplifier sections.

It is noted that the cascaded structure may refer to a series of SOAs, where each SOA sends its output to the input of the next SOA in a daisy chain.

Hence, the modulator may provide amplitude modulation with negligible phase modulation based on a semiconductor optical amplifier (SOA) structure. This modulator allows for an independent control of the optical signal amplitude modulation and phase modulation in a single device.

This approach facilitates optical modulators of low-cost and with low insertion losses, which are a key issue in access and radio over fiber (RoF) DWDM networks. Providing an independent control over the optical phase and amplitude enables generation of optical signals with advanced modulation formats. The solution suggested in particular supports pure amp^¬ litude modulated optical signals or optical signals with in^¬ dependent amplitude and phase modulation. As the structure is an SOA, an overall optical gain can be expected.

In an embodiment, the modulator may be used for or it may fa^¬ cilitate

- OSSB modulation;

- DPSK modulation; - DQPSK modulation;

- CS-RZ modulation;

- substantially chirp-free OOK modulation. In another embodiment, the modulator comprises two, three or four semiconductor optical amplifier sections.

Basically, several SOA sections could be arranged in cascaded structure. Preferably, two SOA sections are used, a first one for the information signal to be applied and a second one for the inverse information signal - preferably multiplied by a factor K - to be applied.

In a further embodiment, the inverse information signal is multiplied by a factor K.

In a next embodiment, the factor K amounts to K = α,₂ / α,ι, wherein α,ι describes a ratio between the amplitude modulation and the phase modulation of a first semiconductor optical amplifier section and 0,₂ describes a ratio between the ampli^¬ tude modulation and the phase modulation of a second semiconductor optical amplifier section.

It is noted that 0,₂, α,ι are also referred to as linewidth en- hancement factors. It is also noted that the modulator may comprise in particular two semiconductor optical amplifier sections .

It is also noted that in case α,ι >> α,₂ and K << 1, the modu- lator does predominately show amplitude modulation and no (or nearly no) phase modulation.

It is also an embodiment that the ratio between the amplitude modulation and the phase modulation of the first semiconduc- tor optical amplifier section and the ratio between the amplitude modulation and the phase modulation of the second se^¬ miconductor optical amplifier section are controlled by, e.g., a bias current or by amplifier design. Said bias current may be a bias DC current.

Pursuant to another embodiment, the semiconductor optical amplifier section driven by bias current of lower amplitude is arranged in prior to a subsequent semiconductor optical amplifier section that is driven by the bias current of higher amplitude. According to an embodiment, the modulator is used in an ac^¬ cess network or in a radio-over-fiber network.

According to another embodiment, at least two subsequent se^¬ miconductor optical amplifier sections are arranged for phase modulation purposes.

Hence, the information signal may be an electrical signal driving the phase modulation stage. In yet another embodiment, at least two subsequent semicon^¬ ductor optical amplifier sections are arranged for amplitude modulation purposes.

Hence, the information signal may be an electrical signal driving the amplitude modulation stage.

According to a next embodiment, the information signal comprises at least one of the following:

- an electrical signal driving the phase modulation

stage;

- an electrical signal driving the amplitude modulation stage .

Hence, the information signal may comprise a combination of an electrical signal driving the phase modulation and another electrical signal driving the amplitude modulation; the electrical signals are applied to at least two semiconductor optical amplifier sections, wherein if the electrical signal is applied to one semiconductor optical amplifier section, the inverse electrical signal (together with the factor K) is applied to another semiconductor optical amplifier section. This allows efficient amplitude modulation or amplitude and phase modulation of an optical signal. Hence, for that purpo^¬ se at least two semiconductor optical amplifier sections are required as combined electrical signals could be applied to any semiconductor optical amplifier section. The problem stated above is also solved by a method for ope^¬ rating an optical modulator comprising at least two semiconductor optical amplifier sections that are arranged in a cas^¬ caded structure, wherein an information signal is applied to one of the semiconductor optical amplifier sections and an inverse information signal is applied to another of the semi^¬ conductor optical amplifier sections.

It is noted that the features described with regard to the modulator apply accordingly for said method. Basically, the method can be used to operate the modulator. In particular, a processing unit can be provided that is arranged such that the modulator is operated accordingly. The processing unit can comprise at least one, in particular several means that are arranged to execute the steps of the method described herein. The means may be logically or physically separated; in particular several logically separate means could be com^¬ bined in at least one physical unit.

Said processing unit may comprise at least one of the follo- wing: a processor, a microcontroller, a hard-wired circuit, an ASIC, an FPGA, a logic device.

According to an embodiment, the inverse information signal is multiplied by a factor K and the factor K results in K = 0,₂ / α,ι, wherein α,ι describes a ratio between the amplitude modu^¬ lation and the phase modulation of a first semiconductor optical amplifier section and 0,₂ describes a ratio between the amplitude modulation and the phase modulation of a second se^¬ miconductor optical amplifier section.

Pursuant to yet an embodiment, the information signal compri- ses at least one of the following:

- an electrical signal driving the phase modulation;

- an electrical signal driving the amplitude modula^¬ tion.

Furthermore, the problem stated above is solved by a communi cation system comprising at least one modulator as described herein .

Embodiments of the invention are shown and illustrated in the following figures:

Fig.l shows a schematic of a semiconductor optical ampli^¬ fier (SOA) section; Fig.2 shows a SOA structure comprising two sections to ob^¬ tain amplitude modulation, wherein an information signal is applied to a first section and the inverted information signal multiplied by a factor K is ap^¬ plied to a second section of the SOA structure;

Fig.3 shows a schematic diagram depicting an operation

scheme of a SOA structure with two sections to obtain amplitude modulation;

Fig.4 shows three schematics of semiconductor optical am^¬ plifiers to obtain controlled AM and PM comprising four, three and two sections.

It is suggested using a multi-electrode SOA structure to ge- nerate a pure AM optical signal or an optical signal with in^¬ dependent AM and PM in particular with an optical gain. Thus, optical signals may be generated with advanced modula^¬ tion formats (e.g., OSSB, DPSK) for access or RoF networks.

Amplitude Modulation only: Conceptual Approach

Fig.l shows a schematic of a semiconductor optical amplifier section 101. The amplifier section 101 induces gain (amplitude) and phase modulation over an input signal Ei_n as a func^¬ tion of an AC-coupled driving signal s (t) . For simplicity reasons, a DC-driving signal (offset) is not shown in Fig.l.

An output signal E_out of the semiconductor optical amplifier section 101 can be described as

E_in(t) corresponds to an electrical field of the opti^¬ cal signal at the input of the semiconductor op^¬ tical amplifier section 101;

E_out(t) corresponds to an electrical field of the opti^¬ cal signal at the output of the semiconductor optical amplifier section 101;

G is an optical power gain;

a describes a ratio between the amplitude modula- tion and the phase modulation (a is also known as linewidth enhancement factor) ; and γ is a rate of cavity loss caused by facet and

waveguide losses.

In case the input signal varies in a scale of time below a SOA recovery time and the input power is low to prevent gain saturation, it can be assumed that the gain is based on the modulating signal s (t) as follows:

The output signal can then be described as:

Fig.2 shows a SOA structure comprising two sections 201 and 202 to obtain amplitude modulation. An information signal m(t) is applied to the section 202 and the inverted informa^¬ tion signal -m(t) multiplied by a factor K is applied to the section 201. The signal at the output of the SOA structure can be described by applying equation (1) to the two sections 201, 202:

Assuming that

and

In order to achieve AM only, the following condition is to be met

In case

It can be shown that equation (6) does not have any phase mo^¬ dulation; furthermore, the optical power is modulated by the information signal m(t) .

In case

The operation of this scheme is illustrated in Fig.3 for square waveforms. As can be seen, at an input Ei_ni of the first section 201 there is no intensity or phase modulation. After the first section (i.e., at an output E_outi of the first section 201) the intensity and phase of the optical signal are modulated with the inverse of the attenuated input data signal. This signal is then fed to the second section 202. The data signal is driving this section, but with a higher amplitude than in the first section 201.

By properly controlling the attenuation in the first section 201 in relation to 0,1/0,2, cancellation of the PM is achieved, but the AM is preserved.

On the other hand, if A^■ m(i) is not negligible (when compared to 1), the data signals driving the SOA can be pre-distorted with a logarithmic function so that the output signal power is still proportional to the data signal.

Amplitude Modulation only: Exemplary Implementation The proposed solution suggests a SOA with two active sections or two cascaded SOA sections comprising distinct linewidth enhancement factors Oi, 02- The linewidth enhancement factors can be controlled by a bias DC current. The relation between linewidth enhancement factor and bias current is described, e.g., in T. N. Nielsen, et al . , "Cancellation of inherent AM in semiconductor optical amplifier phase modulators", in Electron. Lett., vol. 28, no. 3, 30.01.1992, pp. 235-236 or in N. Storkfelt et al . , "Measurement of carrier lifetime and linewidth enhancement factor for 1.5-mu m ridge-waveguide la- ser amplifier", in IEEE Photon. Technol. Lett., vol. 3, no. 7, July 1991, pp. 632-634. For such purpose, a SOA with two electrodes or two cascaded SOAs may be employed.

It is noted that since the SOA bias current is different in the two electrodes, so are the A and D parameters; therefore the K factor can be calculated as follows:

The solution presented herein in particular adjusts the K factor to minimize the AM. The order of the two sections 201, 202 can be changed; howe^¬ ver, preferably, the section driven by the electrical signal with lower amplitude may be the first section, which is bene^¬ ficial in order to reduce signal distortion due to Self-Gain and Self-Phase modulation.

Independent Amplitude and Phase Modulation

It is further suggested generating an optical signal with in- dependent AM and PM. This objective may be met by using a se^¬ miconductor optical amplifier comprising four sections or four cascaded SOAs .

In the first two sections, PM may be conducted as described in T. N. Nielsen, et al . , "Cancellation of inherent AM in semiconductor optical amplifier phase modulators", in Electron. Lett., vol. 28, no. 3, 30.01.1992, pp. 235-236.

In the remaining two sections, AM can be conducted accor- dingly as described above under "amplitude modulation only".

The order of sections performing PM and AM is interchangeable. Nevertheless, the order indicated above may be pre^¬ ferred .

Fig.4 shows three schematics of semiconductor optical ampli^¬ fiers to obtain controlled AM and PM comprising four, three and two sections. A signal m_A(t) is an electrical signal dri^¬ ving the PM and a signal m_B(t) is an electrical signal dri- ving the AM. As a result of this proposal, it is possible to perform AM and PM in the same device, with gain.

The number of semiconductor optical amplifier sections can be reduced from four to three or two as shown in (a) to (c) of Fig.4.

The exemplary implementations (a) and (b) may be preferred, because they may be more cost efficient, as the SOA can be directly integrated with the laser source. Furthermore, amp^¬ litude and phase modulation can be controlled independently. Since semiconductor sections are used, optical gain is expec^¬ ted, which is a key factor for low cost access networks or passive optical networks.

List of Abbreviations :

AC Alternating Current

AM Amplitude Modulation

CS-RZ Carrier-Suppressed Return-to-Zero

DC Direct Current

DPSK Differential PSK

DQPSK Differential QPSK

DWDM Dense Wavelength Division Multiplexing

MZM Mach-Zehnder Modulator

OOK On-Off Keying

OSSB optical Single Sideband

PM Phase Modulation

PSK Phase Shift Keying

QPSK Quadrature PSK

RoF Radio over Fiber

SOA Semiconductor Optical Amplifier