**2.2. Q-switching performance of Thulium-Ytterbium co-doped fiber laser**

Initially, the continuous wave (CW) TYDFL was investigated without using the SA, and the laser threshold was found to be at 1.0 W pump power. When SA is inserted in the cavity, stable and self-starting Q-switching operation is obtained just by adjusting the pump power at a threshold value of 1.6 W, which is higher than that of the CW TYDFL because of the presence of SA in the cavity which increases the loss [2]. **Figure 10** shows the output spectrum of the Q-switched TYDFL at the threshold (1.6 W) pump power. As seen in the figure, the laser operates at 1977.5 nm with an optical to signal noise ratio (OSNR) of around 30 dB.

**Figure 12** shows the pulse repetition rate and pulse width as a function of the pump power. As the pump power increases from 1.6 to 2.3 W, the repetition rate of the Q-switched pulses grows from 18.8 to 50.6 kHz. At the same time, the pulse duration significantly reduces from 8.6 to 1.0 μs as expected. The pulse duration could be further narrowed by optimizing the parameters, including shortening the cavity length, and improving the modulation depth of the MWCNT Q-switcher. An anomalous increase in the pulse width is observed at 1862 mW

**Figure 9.** Setup of the proposed Q-switched TYDFL with MWCNT-PVA-based SA. Inset shows the image of the film

Cladding Pumped Thulium-Ytterbium Short Pulse Fiber Lasers

http://dx.doi.org/10.5772/intechopen.81060

33

pumping, attributed to fiber nonlinearities [21].

**Figure 10.** Output spectrum of the Q-switched TYDFL at pump power 1.6 W.

attached onto a fiber ferule.

**Figure 11(a)** and **(b)** shows the temporal analysis and the corresponding single pulse envelop of a typical Q-switched pulse train, respectively, at a pump power of 1.6 W [2]. The spacing between two pulses in **Figure 11(a)** is around 53 μs, which can be translated to a repetition rate of 18.8 kHz. The corresponding pulse width is around 8.6 μs as shown in **Figure 11(b)**.

**2. Q-switched TYDFL using multi-walled carbon nanotubes passive** 

There are growing interests in compact Q-switched laser sources that operate in the midinfrared spectral region around 2 microns. This is mainly driven by the applications in spectroscopy, communication, material processing, manufacturing, sensing, medicine, and nonlinear optical research [18, 19]. Nowadays, another type of nanotube called multi-walled carbon nanotubes (MWCNT) has been examined in the field of nonlinear optics because its production cost is 50–80% lower than SWNTs [9]. MWCNTs also have higher mechanical strength, higher photon absorption per nanotube, and higher mass density which leads to

We successfully demonstrated, a Thulium-Ytterbium co-doped fiber (TYDF) Q-switched laser using a laboratory made saturable absorber based on MWCNTs implanted in polyvinyl alcohol (PVA) composite for the first time [2]. A homemade double clad Thulium-Ytterbium codoped fiber (TYDF) drawn from a preform which was manufactured based on the modified chemical vapor deposition (MCVD) and solution doping processes is used as a lasing medium. The fabricated MWCNT-PVA film (SA) is attached within the laser cavity by sandwiching it between two fiber connectors. Similarly, a modelocked TYDFL is also demonstrated using

The schematic of the proposed Q-switched TYDFL is shown in **Figure 9**. It is constructed using a simple ring cavity, in which a 15-m long laboratory made TYDF is used for the active medium. An indigenously developed MWCNT-PVA-based SA was used as a Q-switcher. The double clad TYDF was forward pumped by a 905-nm multi-mode laser diode via a MMC.

Initially, the continuous wave (CW) TYDFL was investigated without using the SA, and the laser threshold was found to be at 1.0 W pump power. When SA is inserted in the cavity, stable and self-starting Q-switching operation is obtained just by adjusting the pump power at a threshold value of 1.6 W, which is higher than that of the CW TYDFL because of the presence of SA in the cavity which increases the loss [2]. **Figure 10** shows the output spectrum of the Q-switched TYDFL at the threshold (1.6 W) pump power. As seen in the figure, the laser operates at 1977.5 nm with an optical to signal noise ratio (OSNR) of

**Figure 11(a)** and **(b)** shows the temporal analysis and the corresponding single pulse envelop of a typical Q-switched pulse train, respectively, at a pump power of 1.6 W [2]. The spacing between two pulses in **Figure 11(a)** is around 53 μs, which can be translated to a repetition rate of 18.8 kHz. The corresponding pulse width is around 8.6 μs as shown in **Figure 11(b)**.

**2.2. Q-switching performance of Thulium-Ytterbium co-doped fiber laser**

**saturable absorber**

32 Laser Technology and its Applications

better stability [20].

around 30 dB.

graphene PVA film as a saturable absorber.

**2.1. Configuration of the Q-switched TYDFL**

**Figure 9.** Setup of the proposed Q-switched TYDFL with MWCNT-PVA-based SA. Inset shows the image of the film attached onto a fiber ferule.

**Figure 10.** Output spectrum of the Q-switched TYDFL at pump power 1.6 W.

**Figure 12** shows the pulse repetition rate and pulse width as a function of the pump power. As the pump power increases from 1.6 to 2.3 W, the repetition rate of the Q-switched pulses grows from 18.8 to 50.6 kHz. At the same time, the pulse duration significantly reduces from 8.6 to 1.0 μs as expected. The pulse duration could be further narrowed by optimizing the parameters, including shortening the cavity length, and improving the modulation depth of the MWCNT Q-switcher. An anomalous increase in the pulse width is observed at 1862 mW pumping, attributed to fiber nonlinearities [21].

**3.1. Configuration of the proposed modelocked TYDFL**

self-starting mode locked laser.

**3.2. Modelocking performance of the TYDFL**

pulse train when the pump power is fixed at 1610 mW.

confirms that the laser is operating in anomalous dispersion regime.

Experimental setup for the Thulium-Ytterbium co-doped fiber laser (TYDFL) is shown in **Figure 13** [1]. It uses a double clad Thulium-Ytterbium co-doped fiber (TYDF) as the lasing medium in ring cavity configuration. The TYDF has an octagonal inner cladding to enhance the pump light interaction with the doped core. The core has 5.96-nm diameter with an NA of 0.23. The selected fiber length of 10 m provides more than 90% pump absorption. The fiber type SA device was constructed by inserting graphene PVA film between two ferules. The length of total cavity is set at around 17 m, so that the net cavity dispersion is anomalous for facilitating

Cladding Pumped Thulium-Ytterbium Short Pulse Fiber Lasers

http://dx.doi.org/10.5772/intechopen.81060

35

Modelocking was self-started by increasing the pump power above the threshold of 1487 mW. The modelocked operation was maintained as the pump power is increased up to the maximum power of 1964 mW. **Figure 14** shows the output spectrum of the modelocked

As shown in the figure, the spectrum was centered at 1942.95 nm with a 3-dB bandwidth of 0.08 nm. Without SA, the CW laser operates at 1943.50 nm. This shift of the operating wavelength toward the shorter wavelength is caused by the change in cavity loss by the insertion of SA [1]. Usually, lasers shift toward the shorter wavelength to acquire more gain to compensate for the insertion loss of SA. Moreover, the presence of weak sideband at 1942.5 nm confirms the existence of soliton as shown in **Figure 14**. The presence of conventional soliton

The typical pulse train of the passively mode-locked TYDFL at pump power of 1627 mW is shown in **Figure 15** [1]. The observed repetition rate is 11.76 MHz with a pulse-to-pulse

**Figure 13.** The schematic setup of the modelocked TYDFL employing the fabricated graphene PVA film-based SA.

**Figure 11.** (a) A typical pulse trains and (b) a single pulse envelop of the proposed Q-switched TYFL at a pump power of 1.6 W. It shows a repetition rate of 18.8 kHz and a pulse width of 8.6 μs.

**Figure 12.** Repetition rate and pulse width as a function of 905-nm pump power.
