Transition metal dichalcogenides (TMDs) are successfully applied in fiber lasers for their photoelectric properties. However, in previous work, how to improve the modulation depth of TMD-based saturable absorbers (SAs) has been a challenging issue. In this paper, ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$ SAs are fabricated with the chemical vapor deposition method. Compared with previous experiments, the modulation depths of ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$ SAs with sandwiched structures are effectively increased to 31.25% and 25.69%, respectively. The all-fiber passively $Q$-switched erbium doped fiber lasers based on ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$ SAs are demonstrated. The signal-to-noise ratios of those lasers are measured to be 72 and 57 dB, respectively. Results indicate that the proposed ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$ SAs are efficient photonic devices to realize stable fiber lasers.