The optical emitters used for direct-modulation Laser typesLaser types

Principles A laser diode is a component able of producing radiations by stimulated emissions. In order to maintain alive the stimulation process, a cavity is used, to make the light beam travel several times through an amplifier, and is realized by a feedback loop called resonator or resonating cavity (Fig. 2.8). When the components of the laser are on a same line the cavity is called linear and corresponds to a Fabry-Pérot.

Figure 2.8.: Laser cavity for a Fabry-Pérot

In order to obtain the laser effect, a minimum of electrical or optical energy (pumping) has to be brought in the aim that, at each loop, the gain brought by the amplifier exceeds the losses of the mirrors and of the propagation through the cavity. The quantity of energy generating the gain which exactly compensates the losses is called the laser threshold.

Once the laser effect has started, it is to be noticed that some favored waves propagate in the cavity, and others are attenuated. The favored waves are called longitudinal modes: a longitudinal mode exists each time that the accumulated phase of wave during a round-trip values a multiple of 2·𝜋. In other words when the optical distance of the cavity round-trip corresponds to an integer multiple of the optical signal’s wavelength.

However for systems requiring a single wavelength, a source having a single longitudinal mode is mandatory.

In order to conserve a single mode, the idea is to suppress the undesired modes by using the principle of Bragg reflections, which allows to filter and to let only a single wavelength pass through (Fig. 2.9a).

The using of Bragg gratings in DFB lasers is shown in Fig. 2.9b.

(a) (b)

Figure 2.9.: (a) Bragg grating; (b) DFB structure

Network usage According to [22], Fabry-Perot (FP) and Distributed Feed Back (DFB) lasers can be used at the OLT as well as at the ONUs in optical access networks. While FP lasers exhibit spectral line-width of 3nm, DFB lasers can have line-width as low as 0.001nm at a wavelength of 1500nm [25]. Therefore FP lasers are used primary at the 1300nm wavelength transmission window where the dispersion of S-SMF is low. Thus for bit rates greater than 1.25Gbit/s and distance greater than 10km FP lasers are not recommended in PON networks [22].

Finally cooled DFB laser diodes are preferred. The latter have wavelengths drift due to ambient temperature of 0.1nm degree Celsius. Yet they are more expensive than FP but at the CO, their usage is shared between the users.

Typical launch powers

Tab. 2.2 shows the maximum and minimum specified [17, 21, 31] optical launch powers, with respect to the different optical budget classes, when using a single fiber and bit rates of 2.5 and 1.25Gbit/s respectively in the downstream and in the upstream.

Even if in optical fibers the insertion losses depend upon the working wavelengths, the specified optical budgets apply to the 1490nm central downstream wavelength as well as to the 1310nm central upstream wavelength.

Chirp due to the laser

The chirp can be defined as the instantaneous change of the central frequency 𝑓₀(𝑡) in response to variations in the optical power. In other words, for an amplitude modulated optical wave this results in a residual frequency modulation. This frequency chirp, Δ𝑓, can be expressed as:

Table 2.2.: Specified G-PON network classes and their attenuations (fiber included), launch &

receive powers at OLT and ONU

Network Class A B C B+ C+

Attenuation min [dB] 5 10 15 13 17

Attenuation max [dB] 20 25 30 28 32

OLT TX power min [dBm] 0 5 3 1.5 3

OLT TX power max [dBm] 4 9 7 5 7

OLT RX power min [dBm] -23 -28 -29 -28 -32

OLT RX power max [dBm] -8 -13 -29 -8 -12

ONU TX power min [dBm] -2 -2 2 0.5 0.5

ONU TX power max [dBm] 3 3 7 5 5

ONU RX power min [dBm] -21 -21 -28 -27 -30

ONU RX power max [dBm] -1 -1 -8 -8 -8

Δ𝑓(𝑡) = 1 2𝜋

𝑑𝜑(𝑡) 𝑑𝑡 = 𝛼

4𝜋

(︃ 1 𝑃(𝑡)

𝑑𝑃(𝑡)

𝑑𝑡 +𝜅·𝑃(𝑡)

)︃

(2.6) where𝑃(𝑡) and 𝜑(𝑡) are respectively the instantaneous optical magnitude and phase.

The expression of Δ𝑓(𝑡) can be decomposed in two terms:

1. a transient chirp parameter: it evolves with the time derivate of the optical power and is function of 𝛼 (called linewidth enhancement factor)

2. an adiabatic chirp term: it produces a frequency shift proportional to the instanta- neous optical power and depends upon 𝛼 and 𝜅.

The analysis of the transient chirp term shows a dependency upon the time variation of the instantaneous optical power, and thus —for digital applications— a dependency upon the rise and fall times of the modulating signal.

When the optical source is intensity modulated (low modulation index 𝑚) by a sine wave of frequency 𝑓, a residual phase modulation (factor 𝑝) of each sidebands can be measured [32].

These factors are related as follows:

2𝑝 𝑚 =𝛼

⎯

⎸

⎸

⎷1 +

(︃𝑓_𝑐 𝑓

)︃2

=𝛼

⎯

⎸

⎸

⎷1 +

(︃

𝐼₀ 𝜅 2𝜋·𝑓

)︃2

(2.7) where𝑓_𝑐 is the chirp frequency and 𝐼₀ the bias current of the laser.

Fig. 2.10a illustrates the previous formula. Indeed, for a constant modulating frequency below 1GHz, the 2𝑝/𝑚 ratio factor decreases with bias current. Whereas for modulating frequencies beyond 1GHz, the 2𝑝/𝑚 ratio tends to converge asymptotically, no longer depending upon the bias current.

Finally, Fig. 2.10b shows the evolution of the transient (𝛼) and adiabatic (𝜅) chirp factors with respect to bias current.

(a) (b)

Figure 2.10.: Example of Laser chirp values for different bias conditions, (pictures from [32]

Modulation bandwidths for a direct modulation

The modulation bandwidth of the lasers determines the maximum bit rate for which the laser can be used. For laser diodes (such as DFB lasers), the small signal modulation bandwidth can be approximated [25] by:

𝐵𝑊 ∝^√︁𝐼_{𝐵𝑖𝑎𝑠}−𝐼_{𝑇 ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑} (2.8)

Fig. 2.11a illustrates this relationship. Thus in high-speed transmissions, in order to increase the modulation bandwidth, the laser is biased as much above the threshold current as possible, while remaining below the laser’s relaxation frequency.

According to [33], at bit rates of 10Gbit/s or higher the frequency chirp imposed by direct modulation (the injected current modulates the optical frequency [25]) becomes large enough that direct modulation of semiconductor lasers is rarely used.

For such high-speed transmitters, the laser is biased at a constant current to provide the CW output, and an optical modulator (such as a Mach Zehnder modulator) placed next to the laser converts the CW light into a data-coded pulse train with the right modulation format.

Relative Intensity Noise (RIN)

The stimulated emission of photons in the laser produces a coherent electromagnetic field. However, occasional spontaneous emissions add amplitude and phase noise to this coherent field, even when the laser is biased at a constant current with negligible current fluctuations [25, 33].

The results are a broadening of the (unmodulated) spectral linewidth and fluctuations in its intensity. The latter effect is known as Relative Intensity Noise (RIN).

Since such fluctuations can affect the performance of light-wave systems (demonstrated in

§7.2, p. 112 and 7.3, p. 121), it is important to estimate their magnitude. The fluctuations

(a) (b)

Figure 2.11.: (a) Measured modulation response of a 1.3um DFB laser as a function of modulation frequency at several bias levels;

(b) RIN spectra at several power levels for a typical 1.55um semiconductor laser, (both pictures from [33])

are translated by the RIN figure of the laser (in dBc/Hz) with respect to the average emitted optical power.

Figure 2.12.: PIN photo-detector (schematically)