Laser Opto-Electronic Oscillator and the Modulation of a Laser Emission

*Alexander Bortsov*

## **Abstract**

The autonomous optoelectronic generator (OEO) is considered in the chapter as a source of low-noise oscillations. Differential equations are considered and methods with OEO modulation with direct and external modulation are analyzed. The complexity of both approaches is related to the non-standard way of description of the nonlinear method modulation for the internal (direct) structure and the utilization of the specific Mach-Zehnder modulator for the first stage on external modulation. The purpose of the presentation is to consider the main features of OEO as a low-noise generator. This includes consideration based on the study of differential equations, the study of transients in OEO, and the calculation of phase noise. It is shown that different types of fibers with low losses at small bending radii can be used as a FOLD in OEO. The important role of the choice of a coherent laser for OEO with a small spectral line width is shown. The prospects of using structured fibers with low losses at bends of less than 10 mm in OEO are described. The results of modeling dynamic processes in OEO with direct modulation are presented.

**Keywords:** opto-electronic oscillator, phase noise, optical fiber, QW laser, microwave oscillator

#### **1. Introduction. The opto-electronic oscillator structure**

Development and creation of the compact ultra-low-noise microwave signal sources, which would be impact-resistant, is an important problem of modern radio-physics and radio engineering. Levels of the phase noise spectral density at the microwave source output must be for most of the applications 120 … -170 dB/ Hz at generation frequency 8 … 12 GHz for 1-kHz offset from a carrier. Constructions of these oscillators must sustain the strong mechanical impact loads in 200 … 2000 N/cm and high accelerations up to 2 … 10 g. Geometrical dimensions of the modern signal sources should often be approximately 10х10х10 cubic mm, especially for the satellite applications.

Development and implementation of new compact microwave and millimeterwave oscillators with improved performance would lead to revolutionary jump in radio electronics, perhaps, comparable to discovery of the quantum-dimensional lasers or (as in radio engineering) at arriving of the high-stability quartz crystal resonator. The new type of oscillators called as opto-electroniс oscillator (OEO) described in this paper will permit to use in the mobile communications and in

Internet systems of new radiofrequency channels for information transmission, including 30 … 75-GHz ranges at the low power of transmitters. A number of publications devoted to OEO experimental investigations grows each year [1–8].

Opto-electronic oscillators will undoubtedly find wide application in the fiberoptical communication lines as well as in on-board radar systems on millimeter- and centimeter ranges, in communication systems as low-noise local oscillators in receivers and as a master clock in transmitters, in an optical lidar technology, as sensors of different physical quantities and in many other systems [8–16].

OEO diagrams with the direct modulation (OEO DM) presented in **Figure 1a** and the OEO structural diagram with *external modulation* of optical emission, which is often called as an opto-electroniс oscillator with the Mach–Zehnder modulator (OEO MZ) presented in **Figure 1b.**

Let us consider the case of OEO operation with a small modulation index, and under the condition that the width of the spectral line of the optical laser

#### **Figure 1.**

*Structural diagrams of the optoelectronic oscillator (a) with the direct modulation by current and (b) with external Mach-Zehnder modulator. Laser = the optical quantum generator (the laser or QWLD), MZ = the electro-optical MZ modulator, OA = the optical amplifier, OF = the optical filter, OF = the optical fiber, Pp = the pumping power, PD = the photo-detector, NA = the nonlinear amplifier, F = the RF filter, C = the RF coupler, CH1, CH2 = the optical channels of the MZ modulator.*

*Laser Opto-Electronic Oscillator and the Modulation of a Laser Emission DOI: http://dx.doi.org/10.5772/intechopen.98924*

generationΔ*vL* is much smaller than the radio frequency *f* <sup>0</sup> of the OEO generation: Δ*ν<sup>L</sup>* < < *f* <sup>0</sup>. In this case, the spectrum of the modulated radiation can be represented by several harmonics. We will limit our analysis to the case of two or three optical harmonics, respectively, with frequencies *ν*<sup>1</sup> ¼ *ν*<sup>1</sup> � *f* <sup>0</sup>, *ν*<sup>2</sup> ¼ *ν*0, *ν*<sup>3</sup> ¼ *ν*<sup>3</sup> � *f* <sup>0</sup>. Two of these optical frequencies *ν*<sup>1</sup> and *ν*<sup>2</sup> are spaced from the central optical laser frequency *ν*<sup>0</sup> by the sub-carrier frequency *f* <sup>0</sup>.

**Figure 2a** and **b** shows the analog model of statistical processes in OEO MZ with utilization of the random variables correlator. The correlator structure is described in [**16**]. It consists of the multiplier "\* ", two optical channels with different delays, and the delay cell defining by he delay in the optical fiber. The functional diagram **Figure 2a** and **b** illustrating principles of the correlator method and the frequency

#### **Figure 2.**

*The functional diagram illustrating principles of the correlator method and the frequency discriminator method in OEO with the MZ modulator and in the circuit with direct amplitude modulation at suppression of the one harmonic. a) Diagrams (1–4) of optical frequency selection; b) L = the laser,* Т1M *and* Т2M*, = delay lines have delay times in channels, " + " = (adding), "х" = (multiplication), "\*" = (conjugate operation), "* Ð *" = (integration), F = the low-pass filter, a = the amplifier.*

discriminator method in OEO with the MZ modulator and in the circuit with direct amplitude modulation at suppression of the one harmonic.

### **2. OEO with direct modulation and OEO with the Mach Zehnder modulator**

As in [8–16] when studying noise, OEO is considered here as an optoelectronic system in which oscillations are formed in the optical and radio frequency ranges. The oscillation frequency of the laser is approximately 200 Hz, and the radio frequency of the OAO generation is approximately 10 GHz. A VLD quantumdimensional laser diode is used to generate laser radiation. The positive feedback ring is formed by an optoelectronic circuit consisting of a modulator, an optical fiber, a photodetector, an electronic amplifier, an electronic filter, and a directional coupler.

Fluctuations are formed at the OEO output. Laser fluctuations are of a quantum nature. When using a low-noise amplifier, the phase fluctuations of the laser determine mainly the noise of the OEO output.

#### **3. OEO construction and its operation principle**

**Figure 1a** shows a direct modulated OEO diagram. At the same time, the QWLD laser it works in the mode of amplitude modulation or intensity modulation. In **Figure 1b** the diagram of the OEO with external modulation is presented.

The OEO DM diagram (**Figure 1a**) is formed by a QWLD laser; a single-mode optical fiber (FO); a photodetector (FD); an electronic amplifier (A); an electronic filter (F), for example, based on a dielectric microwave resonator. The OOO MZ diagram (**Figure 1b**), in addition to the QWLD laser, includes elements that form a closed loop: the Mach Zehnder modulator (MZ); a fiber-optic system (FOS) containing an optical filter (OF) and single-mode optical fiber (FO); photodetector (PD), such as a quantum photodiode size; narrow-band RF filter (F), nonlinear amplifier (A), and directional coupler (C). A fiber-optic delay line (RF FODL) is formed by a laser connected in series, OF, FO, and PD (**Figure 1a**), or by a laser connected in series, MZ, OF, FO, and PD (**Figure 1b**). OEO can be considered as a delayed feedback oscillator.

**Figure 1a** and **b** show a laser as a source of optical oscillations, which includes a closed-loop optical amplifier (OA) and an optical filter (OF). We consider the case of the laser radiation modulation mode for single-mode, single-frequency, and linearly polarized optical radiation. When self-excitation conditions are met in such OEO systems (**Figure 1**) generation of microwave range oscillations occurs. A fiberoptic delay line (RF FODL) is formed by a laser connected in series, OF, FO, and PD (**Figure 1a**), or by a laser connected in series, MZ, OF, FO, and PD (**Figure 1b**). OEO can be considered as a delayed feedback oscillator.

**Figure 1a** and **b** shows a laser as a source of optical oscillatons, which includes a closed-loop optical amplifier (OA) and an optical filter (OF). The pump or pumping power *Pp* of the laser is shown conditionally.

We consider the case of the laser radiation modulation mode for single-mode, single-frequency, and linearly polarized optical emission of the highly-coherent laser. When self-excitation conditions are met in such OEO systems, **Figure 1** generation of microwave range oscillations occurs.

In the diagram in **Figure 1**, the Laser is presented by closed into a loop the optical amplifier (OA), the optical filter (OF), which corresponds to the "traveling-

#### *Laser Opto-Electronic Oscillator and the Modulation of a Laser Emission DOI: http://dx.doi.org/10.5772/intechopen.98924*

wave" laser or the fiber-optical laser. The optical pump power *Pp* acts at the active amplifier. If the excitation conditions are met, the laser generates optical oscillations which pass from its output into MZ, then pass via two optical channels with different delays, combine together and through OF and FO acts to the lightsensitive PD area. An effective modulation by MZ is possible in microwave range only for single-mode single-frequency and linear-polarized emission of the highlycoherent laser. Quantum-Well (QW) laser diodes and the fiber-optical lasers with polarizers at their outputs are such emission sources.

The laser is the pump source for the radiofrequency network (**Figure 1b**) closed into a loop and formed by a modulator, an optical fiber, a photo-detector, an electronic amplifier, an electric filter, and a coupler.

As a result of oscillation processes, the spectra are formed with fluctuations having the various nature, but the spectral line width of radiofrequency oscillations is defined by parameters of two oscillating system: the laser and the radiofrequency oscillator.
