**3.2. Modelocking performance of the TYDFL**

**3. Modelocked TYDFL using graphene PVA film as a saturable** 

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

obtained after drying in an oven and is used as SA.

Modelocked Thulium-doped fiber lasers (TDFLs) have attracted intense interest in recent years for a number of potential applications, including atmospheric measurements, material processing, communication, laser radar, biomedical and medical applications, and longer-wavelength laser pumping [22–24]. The graphene PVA film was prepared by mixing the graphene solution in PVA solution. The graphene solution was obtained from the flakes produced using electrochemical exfoliation process. A free-standing graphene PVA film was

**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.

34 Laser Technology and its Applications

**absorber**

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

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 confirms that the laser is operating in anomalous dispersion regime.

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.

fiber laser operates in an anomalous dispersion regime. Moreover, the pulse width can be measured using an autocorrelator or can be calculated mathematically using time bandwidth product (TBP). Since an autocorrelator in the 2-μm range is not available, it is calculated math-

of the optical spectrum is about 0.075 nm (5.96 GHz), the minimum possible pulse width is estimated about 52.85 ps. In addition, the repetition rate of the pulsed laser is also calculated by using the formula c/1.5 L, which gives a value of 11.76 MHz. This is in agreement with the observed repetition rate of this laser as shown in **Figure 16**. Single pulse energy is also calculated at various pump powers by using the measured values of output power of the laser. A maximum output power of 14 mW is observed at 1750-mW pump power. A linear increase in pulse energy is observed up to a pump power of 1750 mW, and with further increase in pump power, it decreases because of energy converted to noise as shown in **Figure 17**. The

pulse profile. Since the 3 dB bandwidth

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

37

Cladding Pumped Thulium-Ytterbium Short Pulse Fiber Lasers

ematically by considering TBP of about 0.315 for sech2

calculated pulse energy is in the range of 756.048–1190.476 pJ.

**Figure 16.** Repetition rate of the modelocked laser at various pump powers.

**Figure 17.** The calculated pulse energy against the input pump power.

**Figure 14.** Output spectrum of the modelocked TYDFL.

**Figure 15.** Typical pulse train for the modelocked TYDFL at a pump power of 1627 mW.

separation of 85 ns, which is in accordance with the cavity round trip time of a cavity length of 17 m. Thus, the well dispersed graphene in the PVA film exhibits sufficient saturable absorption for modelocking operation.

**Figure 16** shows the repetition rate against the pump power. It is found that the repetition rate remained fixed at 11.76 MHz as the pump power increases from 1487 to 1964 mW [1]. The temporal analysis shows that the pulse width of the laser should be less than 9 ns. It is expected that the actual pulse width is much smaller than 9 ns, but due to the limitations of the oscilloscope resolution, it could not be accurately measured.

The total length of the laser cavity is about 17 m, and it consists of 10 m long TYDF and 7 m long single mode fiber (SMF) [1]. The estimated dispersions for TYDF and SMF are −0.083 and −0.034 ps2 /m, respectively, at 1943 nm. Therefore, it is expected that this modelocked fiber laser operates in an anomalous dispersion regime. Moreover, the pulse width can be measured using an autocorrelator or can be calculated mathematically using time bandwidth product (TBP). Since an autocorrelator in the 2-μm range is not available, it is calculated mathematically by considering TBP of about 0.315 for sech2 pulse profile. Since the 3 dB bandwidth of the optical spectrum is about 0.075 nm (5.96 GHz), the minimum possible pulse width is estimated about 52.85 ps. In addition, the repetition rate of the pulsed laser is also calculated by using the formula c/1.5 L, which gives a value of 11.76 MHz. This is in agreement with the observed repetition rate of this laser as shown in **Figure 16**. Single pulse energy is also calculated at various pump powers by using the measured values of output power of the laser. A maximum output power of 14 mW is observed at 1750-mW pump power. A linear increase in pulse energy is observed up to a pump power of 1750 mW, and with further increase in pump power, it decreases because of energy converted to noise as shown in **Figure 17**. The calculated pulse energy is in the range of 756.048–1190.476 pJ.

**Figure 16.** Repetition rate of the modelocked laser at various pump powers.

**Figure 15.** Typical pulse train for the modelocked TYDFL at a pump power of 1627 mW.

the oscilloscope resolution, it could not be accurately measured.

tion for modelocking operation.

**Figure 14.** Output spectrum of the modelocked TYDFL.

36 Laser Technology and its Applications

and −0.034 ps2

separation of 85 ns, which is in accordance with the cavity round trip time of a cavity length of 17 m. Thus, the well dispersed graphene in the PVA film exhibits sufficient saturable absorp-

**Figure 16** shows the repetition rate against the pump power. It is found that the repetition rate remained fixed at 11.76 MHz as the pump power increases from 1487 to 1964 mW [1]. The temporal analysis shows that the pulse width of the laser should be less than 9 ns. It is expected that the actual pulse width is much smaller than 9 ns, but due to the limitations of

The total length of the laser cavity is about 17 m, and it consists of 10 m long TYDF and 7 m long single mode fiber (SMF) [1]. The estimated dispersions for TYDF and SMF are −0.083

/m, respectively, at 1943 nm. Therefore, it is expected that this modelocked

**Figure 17.** The calculated pulse energy against the input pump power.

**References**

[1] Gafsi S. Highly Nonlinear Fiber Characterization for Mid-Infrared Applications. 2016

laser. In: Paper Presented at the Optical Fiber Sensors; 1988

Optical Fiber Communication Conference; 1989

Optics Communications. 1993;**99**(5):331-335

NY, USA: Simon and Schuster; 2002

rp-photonics.com/q\_switching.html

Boca Raton, FL, USA: CRC Press; 2016

2014;**82**(1):15-27

1962;**33**(3):828-829

2014

[2] Snitzer E, Po H, Hakimi F, Tumminelli R, McCollum B. Double clad, offset core Nd fiber

Cladding Pumped Thulium-Ytterbium Short Pulse Fiber Lasers

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

39

[3] Po H, Snitzer E, Tumminelli R, Zenteno L, Hakimi F, Cho N, Haw T. Double clad high brightness Nd fiber laser pumped by GaAlAs phased array. In: Paper Presented at the

[4] Jeong Y, Sahu J, Payne D, Nilsson J. Ytterbium-doped large-core fiber laser with 1.36 kW

[5] Bedö S, Lüthy W, Weber H. The effective absorption coefficient in double-clad fibres.

[6] Sen R, Saha M, Chowdhury SD, Kumar N, Shekhar DP, Ghosh A, et al. High power fiber

[7] Upadhyaya B. High-power Yb-doped continuous-wave and pulsed fibre lasers. Pramana.

[8] Früngel FB. Optical Pulses-Lasers-Measuring Techniques. NY, USA: Academic Press;

[9] Taylor N. LASER: The Inventor, The Nobel Laureate, and The Thirty-Year Patent War.

[10] McClung F, Hellwarth R. Giant optical pulsations from ruby. Journal of Applied Physics.

[11] Paschotta DR. Encyclopedia of Laser Physics and Technology, Q-Switching. https://www.

[12] Webb CE, Jones JD. Handbook of Laser Technology and Applications: Laser Design and

[13] Ngo NQ. Ultra-Fast Fiber Lasers: Principles and Applications with MATLAB® Models.

[14] Digonnet MJ. Rare-Earth-Doped Fiber Lasers and Amplifiers, Revised and Expanded.

[15] Fermann ME, Almantas G, Sucha G. Ultra Fast Lasers, Technology and Applications.

[17] Li D et al. Unidirectional dissipative soliton operation in an all-normal-dispersion Yb-doped fiber laser without an isolator. Applied Optics. 2015;**54**(26):7912-7916

[16] Agrawal GP. Nonlinear Fiber Optics. Fourth ed. Vol. 120. Academic Press; 2007

Laser Systems. Vol. 2. Boca Raton, FL, USA: CRC Press; 2004

2nd ed. Vol. 395. NY, USA: Marcel Dekker, Inc.; 2001

New York, Basel: Marcel Dekker, Inc; 2003. pp. 27-29

continuous-wave output power. Optics Express. 2004;**12**(25):6088-6092

lasers: Fundamentals to applications. Science and Culture. 2015;**81**:319-326

**Figure 18.** RF spectrum of the modelocked TYDFL.

The RF spectrum of the modelocked pulses is also measured, and the result is shown in **Figure 18**. Its fundamental mode peak locates at a frequency of 11.76 MHz and has an SNR of 36.5 dB, which confirms the stability of the modelocking operation [25].
